by Prof. Biscoe, by the common paraboloid alone, it being converted into an immersion instrument by filling its cup with water. These means of illumination, now for the first time available. may act in three distinct ways, one of which is new.

The new method is applicable only to objects mounted dry, and is illustrated by Fig. 123. It has been called, by Rev. S. Leslie Brakey, Internal Illumination. All the light suffers total internal reflection from the upper surface of the slide except that which meets the surface at the points of contact of the object, and the rays thus excepted enter and illuminate the object. Of course the object must lie upon the slide, and beginners are often puzzled by

Fig. 126.

failure, not suspecting or remembering that the object may have been mounted upon the under surface of the cover.

The two other methods are the common opaque illumination and dark-field illumination, which are illustrated together in Fig. 124. as they are usually employed together in practice. They are applicable only to objects mounted in some medium, in which case the top of the cover becomes, optically, the first surface reached by the light after entering the lens. The upper ray in Fig. 124 represents this method of opaque illumination, the ray being reflected by the cover upon the object. The lower ray represents the dark-field effect, the object being illuminated precisely as by the common paraboloid, only the field is darkened not by the obliquity of the rays passing through the cover but by the fact that they are reflected back by it. Hence its greater complete

* This illumination is not exactly represented in the diagram. Most of the light reflected down by the cover is that which passes by the side of the object, and not that which passes through the object. This is shown in Fig. 125, but simplified in Fig. 124.

ness and its applicability to large angular apertures. Its effect is superb with powers as high as of 130° or 140°, especially when used with the binocular.

In practice it is not easy, nor often necessary, to separate the latter two methods. They separate themselves according to the character of the object. With an absolutely opaque object the opaque illumination will alone be accomplished, the dark-field effect (lower ray of Fig. 124) being necessarily suppressed; and success will probably be difficult and only indifferently good. With sufficiently translucent objects the opaque effect would be insignificant, but the dark-field effect easy and excellent. Objects just opaque enough to answer equally to both methods give a confused result, which might possibly be valuable in exceptional cases.

The latest contrivance (Mr. Wenham's, of course) for an immersion illuminator is a glass cylinder half an inch long, one side of which is ground off at an angle of 64° and polished to furnish an internally reflecting surface. The upper end approaches the bottom of the object slide, the interval being filled with water as in the use of an immersion lens, and the lower end is ground to a convex surface whose refracting effect on the pencil of light is supplemented by a plano-convex lens placed below it. In fact we have something like a Wollaston's doublet for a condenser, whose cone of light is twice bent by internal reflection so that its apex is in the position of the object between the cover and the slide. In the diagram (Fig. 126) only a central ray is represented; but in use nearly all the light falling upon the lower lens is brought to a focus on the object, giving an abundance of light and remarkable results with high powers. The apparatus is so mounted on the sub-stage as to rotate around its own focal point as a centre; and excels the former appliances in giving a more intense one-sided illumination, in confining the light to the object. instead of lighting up everything in the neighborhood, and in allowing the slide to be moved or changed with facility.

Immersion achromatic condensers for transparent (bright-field) illumination have not yet received sufficiently extensive trial to ascertain their exact degree of usefulness; but they seem likely to come into use as a means of increasing the available angular aperture of immersion objectives, if not for other purposes.

ON THE CAUSE OF DETERIORATION IN SOME OF
OUR NATIVE GRAPE-VINES, AND ONE OF THE
PROBABLE REASONS WHY EUROPEAN

VINES HAVE SO GENERALLY
FAILED WITH US.

BY C. V. RILEY.*

THE GRAPE-LEAF GALL-LOUSE (Phylloxera vitifolia † Fitch.). The experience of the past year, enables me to add much of interest and importance to last year's account of the above insect. This experience has already been made public in an article published in the "Rural New Yorker," and reproduced in the "Rural World" of St. Louis. I am pleased to know that the views there set forth receive the indorsement of such an experienced and practical man as Mr. Geo. Husmann, the well known grape authority in our State, and editor of the vineyard department of the last named journal.

It is well known that nearly all the varieties of the European grape-vine (Vitis vinifera) have, in the end, proved valueless when introduced and cultivated in the eastern half of the United States. The majority of them grow well at first, and a few exceptional cases might be mentioned where some of them, such as the Black Hamburg and Chasselas, have even fruited successfully for many years, especially when isolated or trained against walls; while

We copy this from Mr. Riley's 4th Report as Entomologist to the State of Missouri, 1872, and have to thank Mr. Riley for the use of the cuts.-EDS.

†This is the specific name by which I last year gave an account of this grape-vine insect; and I employ it again for that very reason, and for the further reason that it is the name most familiar to the American reader. I have already given my opinion (3rd Rep. p. 95, note) that though the name is objectionable, it ought perhaps to be retained. It is doubtful, however, whether many other entomologists will agree with me; and while I believe in carrying out the "law of priority" to its fullest extent, consistent with reason, there are many cases where it must give way to that of "accord." The present is, perhaps, just such a case; for aside from the technical objection, Dr. Fitch knew so little of the insect's true characters, when he named it, that he cannot be said to have described it, and did not refer it to its proper genus which was already erected to receive it. His name will, therefore, doubtless give way to that of Phylloxera vastatrix, which Planchon first gave to the root-inhabiting form, and which has generally been recognized abroad. The same may be said of Westwood's name Peritymbia vitisana, which was also proposed for the same insect in 1868, in a communication to the Ashmolean Society of Oxford, England.

they more generally do well when isolated in cold houses. But the general truth of the first statement holds good. It is also well known that some of our native vines, which for a while were universal favorites on account of their productiveness, vigor and other excellent qualities, have of late years sadly deteriorated. Among such the Catawba was for a long time the popular grape; but its cultivation is now entirely abandoned in many parts of the Mississippi Valley, and even at Hammondsport and other parts of New York, and Nauvoo, Illinois, where it is still largely cultivated, I learn from experienced grape-growers that it is fast on the decline.

this failure, has been attributed to a variety absence of anything definite and ascer

Fig. 127.

This deteriorationof causes, for in the tainable to keep it within bounds, the speculative turn of our minds is sure to have full scope, and grasping at every shadow of probability, leaves no possible theory unsearched. As in all such cases, also, the mind gets lost in, and is satisfied to vaguely rest with, the theory least provable; and to some occult and mysterious change of climate we are at last satisfied to attribute the change though, if the meteorological records were carefully examined, they would probably show no difference in the mean annual condition of our climate during the past half century.

It is very natural to suppose that vines of European origin should be less hardy in this country than our native varieties, that as in the case of the Spanish Chestnut, the English Gooseberry, etc., etc., there is something in our climate which precludes their flourishing as well here as there. I would by no means deny that such is the case, for it is this very comparative tenderness which predisposes them the more to the destructive agent of which I am about to speak. Yet when we consider that in some

parts of Europe, where the Vine flourishes, the extremes of heat and cold are as great as here; that we possess a great variety o soil and climate, and that by covering and other modes of protection in winter, we may, where necessary, counteract the rigor

the fact that our native varieties, if free from the in

sect which forms the subject of this article, usually do well when cultivated in Europe, and further that the Vitis vinifera is not a native of

Europe, but of western Asia.

The above reflections are of a general character, but apply more particularly to the great State of Missouri,

Grape Gall-louse; a, winged female; 6, her tarsus, c, egg, d, the which is admitte i

newly hatched gall-inhabiting type, ventral view; e, dorsal view; f,

section of a gall; g, the tubercled root-inhabiting form; h, the to be, in many

mother gall louse at the height of her fertility, ventral view; i, dor

sal view; j and k, differently veined wings of the Oak Phylloxera parts, eminently

from Europe. All these figures are greatly enlarged, the natural size being shown by the hair lines at the side of each.

soil and climate, to the cultivation of the Vine.

adapted, both by

One of the reasons why the European vines do well in California, outside of and beyond the more favorable clime in that portion of the continent is, no doubt, because the insect which here affects them, like many other species common on this side of the Rocky