Photograi)hing Histological Preparations hj Sunlight. 177 



surface would, I suppose, answer the same purpose; but such 

 mirrors are not permanent, and are troublesome to keep in order 

 while they last. Moreover, if the prism is used only for this pur- 

 pose, a very small and cheap one will answer, since a pencil half an 

 inch in diameter is all that is required. Such a small right-angled 

 prism is furnished with most large microscopes, and can readily be 

 mounted outside the brass disk so as to answer the special purpose 

 indicated. For aU those objects which require the large condensing 

 lens to avoid diffraction and interference, a common glass mirror 

 will answer well enough. For lower powers than two hundred 

 diameters, however, the ordinary mhror will often be found to 

 reflect too much light, and the image on the cardboard screen will 

 be found too brilliant to be conveniently observed for any length of 

 time. In such cases a piece of plain unsilvered plate glass may be 

 substituted for the mui'or. The greater portion of the solar light 

 passes through it and is lost, but enough is reflected to make pic- 

 tures of four hundred diameters in from two to three seconds expo- 

 sure, and these pictures have all the quahties of those made with 

 ordinary mirrors. I have tried instead to diminish the light by 

 absorbing a part, using for this purpose an ammonio-sulphate cell 

 of considerable thickness, but find that this plan diminishes the 

 contrast and definition of the image, which is not the case when a 

 muTor of simple plate glass is used as above described. 



With regard to the management of the plate-holder, the appa- 

 ratus for focussing, and other accessory arrangements, I need only 

 say that I employ for the solar light the same simple plan which I 

 have described in full in my reports on the use of artificial lights in 

 photo-micrography. 



Since making the experiments which have led to the foregoing 

 results I have modified my method of dealing with the electric hght 

 in photographing the tissues. I first render the divergent pencil 

 proceeding from the carbon points as nearly paraUel as possible by 

 means of the condenser, usually supplied with electric lamps for 

 this purpose, and then introduce into the parallel pencil, instead of 

 a ground glass, the very same condensing lens described above for 

 the process with solar light. The image is received primarily on a 

 cardboard screen, and the remaining details do not differ from what 

 has been related above. The time of exposure does not exceed a 

 single second for four hundred diameters, and the sharpness of the 

 pictures exceeds any of my former results. Indeed, with this new 

 arrangement I must say that the electric light appears to me to 

 retain the apparent superiority over sunlight, remarked in my paper 

 on the use of this method of illumination in photo-micrography, at 

 least in the case of all those objects which in themselves possess but 

 little contrast. For well-made tissue preparations, however, I find 

 the best work I can do with the electric light, so similar to the best 



