ZOOLOGY AND BOTANY, MIOROSOOPY, ETC. 915 



blood of yellow fever. This has been demonstrated by the author 

 in the manner indicated for the disease in question. 



Pollen-grains. — Not all of these objects which it would be most 

 desirable to photograph are suitable objects to be photographed by 

 transmitted light, for the reason that the bright yellow colour and 

 comparatively large size of some render them practically opaque. 

 Doubtless this difficulty in the case of pollen-grains, the deeply 

 coloured spores of fungi, &c., can be overcome by special methods of 

 preparation, — the use of decolorizing agents, mounting in media of 

 high refractive index, &c. The limits of the author's volume do not, 

 however, permit him to go very extensively into details with reference 

 to the preparation of objects, even if the technique were completely 

 worked out, which is far from being the case. The general state- 

 ment may be made, however, that objects which, in water, are not 

 sufficiently transparent for photography, should be mounted in media 

 having a higher refractive index, of which the most useful are 

 glycerin and Canada balsam. 



Some pollen-grains swell up and the membranous envelope is 

 ruptured when they are immersed in water. For this reason, as well 

 as for that already given, glycerin is commonly a more suitable fluid 

 in which to mount them. When first placed in glycerin, the cell- 

 wall becomes collapsed from exosmosis of the watery contents ; but 

 after a time the natural form is recovered by endosmosis and the fluid 

 within and without is of the same density. 



To prevent the trouble arising from the presence of air-bubbles, 

 which are apt to adhere tenaciously to the pollen-grains, it is best to 

 immerse them first in alcohol, as recommended for the spores of fungi. 

 A drop of alcohol is placed in the centre of a glass slide, and the ripe 

 anthers, held in slender forceps, are brought into contact with it ; the 

 pollen is detached, and falls to the bottom of the drop. A little 

 agitation of the slide causes it to be distributed in a stratum consist- 

 ing of a single layer of cells. When the alcohol is nearly evaporated, 

 a drop of glycerin is put in its place, the thin cover is applied, and 

 the superfluous fluid removed with bibulous paper. 



Plant Hairs. — Some ingenuity will have to be exercised in pre- 

 paring objects of this kind for photography. Hairs that are closely 

 applied to the surface of the leaf may be photographed in situ by 

 mounting the epidermis, or by reflected light. Others will require to 

 be detached, and may be shaved off with a razor, and mounted in a 

 very shallow cell in water or in glycerin. It is always desirable to 

 obtain a field in which the objects do not overlie or cross each other ; 

 and with long plant hairs, like cotton, this is not an easy matter 

 unless they are carefully arranged one by one. A good plan both 

 for long plant hairs and animal hairs is to place several side by side 

 on a dry glass slide, fixing the ends to the edges of the slide with 

 sealing-wax. When they are adjusted in position the central portion 

 is wot with alcohol, then with water, and finally with glycerin, if it 

 is to be used. A thin glass cover is then applied. 



Aninvd TIairs. — A scries of photo-micrographs of animal and 

 vegetable hairs would bo extremely interesting and instructive. 



