MAMMALIAN. 67 



into alcohol, by which means the specimens are hardened, without becoming so 

 brittle as is often the case if they are kept for a long time in a solution of chromic 

 acid or of bichromate of potash. I have also continued the use of copal-varnish in 

 the place of Canada balsam as recommended in a former 'paiper (Spinal Cord, 1861), 

 as it still seems to me on many accounts more advantageous than the balsam. 



The preparation of specimens for photographic use required some modification of 

 the methods employed, since here the object in view was to obtain as much contrast 

 of structure as possible, rather than that extreme transparency required for the use 

 of higher powers. The sections, for this purpose, were immersed for a short 

 time in very strong alcohol, and after careful washing placed in chloroform, where 

 they shortly become semi-transparent ; they are then placed on a slide on which a 

 couple of drops of copal-varnish have been put, so that the section lies on the surface 

 of the varnish : as the chloroform evaporates the varnish takes its place, and the 

 section is kept soft for about twenty-four hours by adding at intervals either varnish 

 or chloroform, or both, according to the degree of transparency required ; a little 

 practice only being necessary to attain any desired residt. The varnish is then 

 slightly softened by warming the slide over a lamp and the preparation covered 

 with a thin glass as usual. 



The methods employed in photographing the specimens were simple, and wiU 

 readily be understood by those versed in the details of ordinary photographic 

 manipulation. My apparatus consisted of a brass adapter, the tube of which fitted 

 closely into the body of the microscope (Smith and Beck's first class), so that 

 after removing the eye-piece and draw-tube I was enabled to attach the micro- 

 scope very firmly to a common photographic camera. After a variety of experi- 

 ments with different sources of illumination, I fomid the direct sunlight the only 

 one on which I could rely with any degree of certainty, and although it wiU. often 

 appear that much time is lost in waiting for an entirely imclouded day, stiU, so far 

 as my own work was concerned, I found that I lost much more time in endeavoring 

 to work in imcertain weather. The common plane mirror may be used for reflect- 

 ing the sunlight, or what is still better, the right angle prism which accompanies 

 Smith and Beck's microscope. The objective with which the accompanying photo- 

 graphs were taken was a three inch, and I was able to enlarge the field of illumina- 

 tion to a considerable extent, by introducing directly behind the stage, as suggested 

 by my friend Prof. Rood, a double convex lens, the focal distance of which 

 measured a little less than the distance between the object and the back diaphragm 

 of the objective; the exact focal length of the lens is, however, practically of little 

 importance, and by diaphragming the lens to some extent, the central spot of light, 

 shoidd one appear, may be removed without sensibly diminishing the field. What- 

 ever the power used may be, whether high or low, too much care cannot possibly 

 be expended on careful and perfect illumination, not only in obtaining the greatest 

 amount possible, but also in so modifying it according to the character of the object 

 as to obtain the greatest degree of contrast between different parts, and this not 

 only visually but also actinically, which of course is only to be determined by care- 

 ful and repeated experiment. 



The Wet Process Avas the one used for the majority of the photographs, the Dry 



