the light is twice refracted and reflected, the loss of light is much less than one would expect. With the microscope, the prism is placed right above the objective, and the instrument may be used

in any inclined position. A pair of such prisms might be used also for an erecting binocular microscope, of which the two bodies have the same inclination to the stage.

Fig. 1 shows the front and profile of the prism. The projection of the front is a square, that of the profile an isosceles triangle.

Fig. 2 is a view from above. The rays A, B and C of figs. 1 and 2 are the identical ones, their dotted parts are the projections of the rays inside of the glass, and their course may be readily followed in the profile, fig. 1, where the upper ray, A, emerges as the lower one, and the lower ray, C, as the upper one.

As the ray A enters in the perpendicular line above the lower

edge, it will not be reflected out of its plane, while the rays B and C, entering the left side of the prism, reach the inclined faces, from which they are reflected to the opposite lower one, and are changed in their course to the right, from here again reflected, to emerge at the corresponding opposite point. Fig. 3 is a perspective representation of the prism. - Journal of the Franklin Institute.*

STEPHENSON'S BINOCULAR.- Mr. Stephenson has reported to the Royal Microscopical Society, some improvements in his erecting binocular microscope. The lower prisms by which the light is divided and the image laterally inverted, are made smaller than before and placed nearer to the back combination of the objective. They are now made .68 in. long, .412 in. wide, and .2 in. thick; and are inclined to each other at an angle of 44°, making the angular divergence of the bodies 91°, and the distance of the point towards which the eyes converge nearly fifteen inches. The prisms of this size are mounted in a small tube which projects beyond the nozzle of the instrument, and into the mounting of the objective nearly to the posterior combination of glasses. The quantity of glass in the prisms is greatly reduced, and powers as high as in. may be easily used. The standard length of body is secured and also an easy convergence of the eyes.

By altering the angle of inclination of the bodies to the perpendicular from 75° to 66° the use of a Nicol's prism as an analyzer, which is quite unsatisfactory in a binocular, is easily dispensed When polarized light is used the box containing the upper prisms (the original upper prism being now made in two parts) is withdrawn and an analyzing plate of highly-polished glass substituted, reflecting the light at the polarizing angle of 562°, and securing more light and improved definition.

If desired, the upper prisms may be likewise replaced by a plane mirror silvered by the beautiful process employed by Mr. Browning in the manufacture of his reflecting astronomical telescopes. Thus we get rid of all the glass and of two surfaces, but obtain an incomplete reflection and a less permanent reflector.

OPAQUE ILLUMINATION UNDER HIGH POWERS.-H. A. Johnson, M.D., President of the State Microscopical Society of Ill., communicated to that society another method of producing this difficult effect. He employs Prof. H. L. Smith's plan of making

*We are indebted to the Journal for the use of the above cuts.

the objective its own illuminator; but instead of the silvered reflector, thin glass disc, or reflecting prism, used by Prof. Smith and subsequent experimenters, he employs the binocular prism of Wenham's binocular. A beam of light reflected down the oblique body of the Wenham binocular, the prism being in position as for ordinary use, is of course condensed by half the objective upon the object in its focus; while the other half of the objective and direct body of the instrument is used at the same time as a monocular for viewing the object. A portion of the field is intensely illuminated, and definition is said to be better than with other arrangements.

COLLINS' LIGHT CORRECTOR.-Mr. Collins, of London, has introduced a modification of Rainey's Light Modifier, to correct the glare of too intense light or the yellowness of gas-light, lamp-light, etc. A thin plate contains a rotating wheel with four apertures one of which is empty while the others present in turn a ground glass and two shades of blue glass. This contrivance, an illustration of which may be seen in the advertisements in" Science Gossip," is always ready for use, being simply laid, under the object, upon the stage of any microscope; though for use with an achromatic condenser it is somewhat better to have the correcting plate, whether of ground or blue glass, below the condenser. The new form of Light Corrector may be obtained of Miller Bros. whose address is given elsewhere in this number of the NATURALIST.

MEASUREMENT OF ANGULAR APERTURE.- For those students whose stands have not a graduated rotating base, Prof. T. D. Biscoe recommends a plan especially applicable to the larger form of achromatic condensers which cannot be as conveniently arranged for the method advised by Dr. Carpenter. He places the combination horizontally, with a gas flame several feet distant behind it to furnish nearly parallel light, holds a card across the centre of the front lens so as to bisect the cone of light, and with a fine pencil marks the edges of the illuminated portion of the card. A common protractor is used to measure the angle of the lines thus drawn. Though chiefly useful to measure the eye-piece form of achromatic condensers (Webster's condenser, etc.) as actually used with diaphragm, etc., in position, the method may be sometimes convenient for objectives of low power. Prof. Biscoe believes it can be applied to objectives as high as in. of 110° and be reliable within one or two degrees.

ORGANISMS IN CHICAGO HYDRANT WATER.-The "Lens" publishes an interesting note on the subject by H. H. Babcock of Chicago. While the Chicago river flowed into Lake Michigan, pouring into it a slow current of almost stagnant water saturated with organic matter, the hydrant water used in the city, taken from the lake at a point two miles from the shore and where the water is forty feet deep, contained an abundance, though a greatly varying quantity, of organisms, such as diatoms, etc. When the current in the river was reversed, so as to flow from the lake, the vegetable and animal forms mostly disappeared from the hydrant water; and the improved sanitary condition of the city, generally attributed to the absence of the former noxious effluvia from the river, may be due in part at least to the increased purity of the drinking water from the lake. The writer at first suspected that the water of the river had previously directly contributed the organisms found in the water from the "crib," but afterward was led to conclude that the change in the direction of the river only acted by changing the direction of the currents from other sources in the lake, thereby leaving the crib in purer water than before. Much more investigation of this kind ought to be immediately accomplished in view of the growing tendency, if not the imminent necessity, of supplying to our cities water from lakes and large rivers.

RECORD OF NEW FUNGI.-E. C. Howe of New Baltimore, N. Y., has recently discovered and described several new species of Fungi as follows:- Uncinula luculenta, U. Americana, Microsphora sparsa, M. finitima, Pestalozzia Zabriskiei, P. insidens, and Phragmidium fallax.

PODURA SCALES. American microscopists experience great difficulty in obtaining a satisfactory supply of these objects. The English Journals are agitating the same subject, and there seems to be some mystery as to the source of the familiar "test" scales. Mr. McIntyre suggests, not confidently, that they may be developed upon the common Lepidocyrtus curvicollis late in life. Mr. Joseph Beck states that no amount of age will develop them upon the common form of this insect, though they belong to an insect entomologically identical with it but having a different habitat: while E. G., of Matlock, recollecting that the late R. Beck told him that he found the Podura in the rock work in his mother's garden, hunted for them in a cellar and found a lead-colored species which yielded scales equal to Mr. Beck's. He admits, however,

that only one slide in twenty, perhaps one scale in five thousand, was of any use. In his slide prepared by Mr. R. Beck there are only two well marked scales, which appear to be the exception and not the rule. Dr. Josiah Curtis, of Knoxville, Tenn., has been successful in collecting species of Lepisma, "Podura," etc.

THE STUDY OF DIFFICULT" DIATOMS.- Mr. J. Edwards Smith of Ashtabula, Ohio, writes the following note which is suggestive of the experience of many microscopists who study the finer dia toms without making a specialty of them.

"I find that Pritchard describes Navicula cuspidata as having close transverse striæ. I have slides of this diatom, mounted dry and in balsam, and with my Tolles' dry, or wet, have no difficulty in showing longitudinal striæ, far finer than the transverse; indeed when mounted dry both sets can be plainly seen at once. Is there anything remarkable about this?

I notice that Möller places Navicula crassenervis as the 18th on his Probe Platte'. I fail to discover striæ on this diatom either in balsam or mounted dry. Pritchard says they are sometimes wanting. The objectives above named show the fine lines in No. 17 (Cymatopleur alliptica) very nicely, but fail on Nos. 18, 19 and 20. Our friends here are interested to know what others are doing with this 'Probe Platte.' Has Amphipleura pellucida been resolved in balsam? When, by whom, and with what lenses?" ANS.-Nothing can be more certain than the uncertainty of what has been known, in regard to the markings and structure of diatoms. It would be rash at present to pronounce any shell free from markings, and would probably be repeating a mistake too often made already. Different shells of the same species vary greatly in "difficulty," but no explanation or excuse is necessary for a dry failing to go entirely through the series on the testplate. It is by no means easy, though sometimes possible, to resolve A. pellucida in balsam with dry lenses and ordinary illumination. With immersion lenses, however, and monochromatic illumination, there is no difficulty in accomplishing this with objectives of Tolles, Wales, Powell and Lealand, Beck, Hartnack, and Nachet, and doubtless several other makers. About two years ago Count F. Castracane resolved and photographed A. pellucida in balsam, with French lenses and monochromatic sunlight. For a method by which this can be easily accomplished see Dr. Woodward's paper in the NATURALIST for April last.