since assured, by the approval of many of those who have most successfully employed the microscope in physiological investigations: The superiority in resolving power possessed by object-glasses of large angular aperture is obtained at the expense of other advantages. For even granting that there is no sacrifice of that most important element, defining power (which can only be secured, with a very wide angle, by the utmost perfection in all the corrections), yet the adequate performance of such a lens can only be secured by the greatest exactness in the adjustments. Only that portion of the object which is precisely in focus can be seen with an approach to distinctness, everything that is in the least degree out of it being imbedded (so to speak) in a thick fog; it is requisite, too, that the adjustment for the thickness of the glass that covers the object, should exactly neutralize the effect of its refraction; and the arrangement of the mirror and condenser must be such as to give to the object the best possible illumination. If there be any failure in these conditions, the performance of a lens of very wide angular aperture is very much inferior to that of a lens of moderate aperture; and, except in very experienced hands, this is likely to be generally the case. Now to the working microscopist, unless he be studying the particular classes of objects which expressly require this condition, it is a source of great inconvenience and loss of time to be obliged to be continually making these adjustments; and a lens, which, when adjusted for a thickness of glass of roo", will perform without much sensible deterioration with a thickness either of " or of To", is practically the best for all ordinary purposes. Moreover, a lens of moderate aperture has this very great advantage, that the parts of the object which are less perfectly in focus can be much better seen; and therefore that the relation of that which is most distinctly discerned, to all the rest of the object, is rendered far more apparent. Let me remind you, further, that almost all the great achievements of microscopic research have been made by the instrumentality of such objectives as I am recommending. There can be no question about the large proportion of the results which continental microscopists may claim, in nearly all departments of minute anatomical, physiological, botanical, or zoological investigations, since the introduction of this invaluable auxiliary; and it is well known that the great majority of their instruments are of extremely simple construction, and that their objectives are generally of very moderate angular aperture. Moreover, if we look at the date of some of the principal contributions which this country has furnished to the common stock, such as the 'Odontography' of Professor Owen, the Researches into the Structure of Shell' carried out by Mr. Bowerbank and myself, the Physiological Anatomy' of Messrs. Todd and Bowman, the first volume of the Histological Catologue' by Prof. Quekett, and the 'British Desmideæ' of Mr. Ralfs,-we find sure reason to conclude that these researches must have been made with the instrumentality of lenses, which would in the present day be regarded as of very limited capacity.-I hope that, in these remarks, I shall not be understood as in any way desirous to damp the zeal of those who are applying themselves to the perfectionizing of achromatic objectives. I regard it as a fortunate thing for the progress of science, that there are individuals whose tastes lead them to the adoption of this pursuit; who 6 stimulate our instrument makers to go on from one range to another, until they have conquered the difficulties which previously baffled them; and then apply themselves to find out some new tests, which shall offer a fresh difficulty to be overcome. But it is not the only, nor can I regard it as the chief work of the microscope, to resolve the markings upon the Diatomaceæ, or tests of the like difficulty; and although I should consider this as the highest object of ambition to our makers, if the performances of such lenses with test-objects were any fair measure of their general utility, yet as I think that I have demonstrated that the very conditions of their construction render them inferior in this respect for the purposes of ordinary microscopic research, I would much rather hold out the reward of high appreciation (we have no other to give) to him who should produce the best working microscope, adapted to all ordinary requirements, at the lowest cost." Notwithstanding the approval of those, as Dr. Carpenter says, "who have most successfully employed the microscope in physiological investigations," I do not hesitate for a moment to declare, that nothing could be more pernicious to the best interests of science than these remarks. It is unfortunate that such mistaken views should be displayed on this subject, where so great confidence has been placed,-by one, too, whose elementary works on physiology have raised the belief, among many, that he is perfectly conversant with those very tissues which require the nicest and most rigid microscopical investigation. The illustrations which I have given of the great value of highly corrected lenses in the study of minute structures, are sufficient, I think, to refute these views; but I would like to say a few words more in conclusion, especially in reference to the general relations of microscopical investigations to other departments of natural history. To say that objectives with a wide angle of aperture and a flat field, are needed for only a few bodies, such as test-objects, like the Diatomacea and other known difficult subjects, is to ignore the whole great department of histology, and by that to refuse physiology one of the most important aids; in fact, an aid which, with the help of better microscopes, in future, is likely to take the lead in the determination of the laws of animal and vegetable life. I am well aware that the study of histology has been pursued with the ordinary instruments, of the German pattern, in a great measure; but knowing what these have done both in Europe and in this country, and having discovered, by a few glimpses, how much more, and how much better, we might have done, had we possessed one of these highly finished instruments, I can confidently assert, that it is a grave error to tell opticians they had better devote themselves more particularly to the improvement of the ordinary instruments, and let their transcendental corrections of widely gaping objectives serve in the mean while as playthings for curious amateurs. But it is a still more serious mistake to say to students, that an instrument which performs under a variety of circumstances "without much sensible deterioration" is practically the best for all ordinary purposes. So thought Ehrenberg, and yet we all now know what curious mistakes he made. Embryology, too, comes under this proscription; for any one who has attempted to trace the development of animals, especially the lower forms of life, must know that it is impossible to separate the study of their cellular structure from the investigation of their organs. I cannot more fittingly conclude this communication, than by quoting, by Mr. Spencer's leave, a portion of a recent letter of his to me. He says: "It seems to me that there is every reason to hope much from the earnest application of high powers with large angles. So blind and inveterate has been the prejudice in favor of low powers and small angles, in histology, that younger and less prejudiced microscopists have a comparatively untrodden path before them. Every day's thought convinces me more and more deeply of the radical mistake that has been made in this direction. I have recently been making some observations and experiments with low angles on certain well-known structures, and have in several instances been struck with a blank astonishment at the utterly false, though apparently reliable, results obtained. It happens, too, that the physical and optical characters of those tissues which, oftener than any others, are the subjects of your study, are precisely such as will lead to the most frequent errors; and if you do not find that many a blunder has been made in their study, heretofore, I shall be greatly surprised." ART. V.-On Brewsterite; by J. W. MALLET. Two analyses of the mineral species Brewsterite are on record, those of Connell* and Thomson,t both made many years ago. The results were: *Edinb. N. Phil. Jour., No. XIX, p. 35. + Outlines of Mineral. Geol. and Min. Anal., vol. i, p. 348. It is strange that in Thomson's Outlines of Min., Geol., &c., the analysis of Connell is given with altogether different figuresthus: Dr. Thomson remarking at the bottom of the page that the specimen analyzed by himself consisted of fine crystals carefully selected, while that examined by Mr. Connell was a mixture of amorphous and crystallized mineral. The method for the separation of baryta, strontia, and lime, employed by Connell-probably by both analysts-namely, the solution of nitrate of lime and afterwards of chlorid of strontium, in alcohol-has given place to more reliable processes, and on this account a repetition of the analysis might be desirable; but it becomes still more so when the close analogy of brewsterite to heulandite is considered. The two species should in all probability have the same general formula, and this has in fact been assigned to them in Dana's Mineralogy, but with the formula for heulandite these older analyses of brewsterite do not very well agree. I have recently analyzed some fine specimens, from the original locality Strontian in Argyleshire, Scotland-and the results appear fully to establish the chemical as well as crystallographic relationship with heulandite. The mineral formed crusts of minute crystals upon the surface of gneiss sometimes these crusts could be detached from the rock by careful blows, but in general they adhered very firmly. Some of the crystals were an eighth of an inch in length-most of them were much smaller. The following measurements were obtained-using the lettering of Dana. 0:175° 49'-175° 53'-175° 55' I: I=136° 13'. 0:1-2()=157° 23'-157° 17'-157° 20′-157° 22′. I: i-i=112° 13'-112° 17'-112° 12'. The spec. grav. was found = 2·453. For analysis the crystals were carefully broken off, and picked clean from any dust of the accompanying rock. In one case, the mineral was fluxed with carbonate of soda, so as to ensure perfect decomposition, and consequent purity of the silicic acid SECOND SERIES, VOL. XXVIII, No. 82.-JULY, 1859. weighed; the other specimens were treated directly with hydrochloric acid, which seems of itself to be capable of effecting complete decomposition. The baryta was precipitated by hydrofluosilicic acid,* and the relative amounts of lime and stroutia were determined indirectly, by weighing the mixed earths first as sulphates and then as carbonates. The following are the results obtained (1) (5) Mean. Atoms. Silica, 54.42 1.209 4.08 •296 1. .... *08 trace Baryta, Strontia, 8.79 9.20 8.99 178804-1′08 .... Lime, Analysis (4) was spoiled by an accident; and in (3) the determination of the earths was abandoned on ascertaining the ne cessity for the removal of ammoniacal salts before precipitating baryta (vid. note), a precaution which had not been taken in this case. The silicic acid, alumina, protoxyds and water are clearly present in the ratio 4:1:1:5, giving the formula (BaO, SRO, CaO), SiO3+Al2O3, 3SiO+5HO. The atomic relation between the lime, baryta and strontia is near 1:2:4. * In examining the precautions incident to this mode of determining baryta in the presence of strontia or lime, I have found no notice taken in any work on chemical analysis of the solvent effect of ammoniacal salts upon silico-fluorid of barium. Fresenius states that the latter dissolves in 3400 to 3800 parts of water, and in 640 to 733 parts of water acidified by hydrochloric acid, but does not mention salts of ammonia. I digested pure silico-fluorid of barium in the cold, with frequent stirring, for forty-eight hours—(a) with a saturated solution of chlorid of ammonium, (b) with the same solution diluted with twice its volume of water. The fluid was in each case filtered off perfectly clear, 100 cubic centimetres were measured, and the baryta was determined as sulphate. (a) gave 1942 grm. of BaO, SO, 2338 grm, of BaF, Si F. Hence 1 part of the latter salt dissolves in 428 parts of a saturated solution of sal-ammoniac. (b) gave 1409 grm. of BaO, SÔ, of the diluted solution. 1697 grm. of BaF, Si F,, or 1 part in 589 The necessity of removing ammoniacal salts from a fluid in which baryta is to be determined as silico-fluorid is sufficiently obvious. |