1440 



Journal of Applied Microscopy 



ceed as follows : By the aid of the mechanical stage I carefully review the 

 entire slide, noting the location of every suitable specimen of the species 

 desired, of all species not yet photographed, and their degree of perfection. 

 Such specimens should lie in optical contact with the lower side of the cover- 

 glass, be clearly marked and free from foreign matter. These observations are 

 recorded with the readings of the mechanical stage, and by 

 use of the record at any subsequent time the desirable 

 specimens may be turned to readily and photographed with- 

 out loss of time. This search work may be done by day- 

 light or, what I consider an exceedingly good substitute, by 

 means of the acetylene lamp with a rather dense ray filter 

 interposed. The filter, which has given me excellent results, 

 is simply a fiat eight-ounce bottle filled with a fluid com- 

 posed of 175 grams of copper sulphate, 17 grams potassium 

 bichromate, 2 c. c. sulphuric acid and 500 c. c. of water. 

 The color is restful and agreeable to the eyes, and the 

 density is not sufiicient to interfere in any serious way with 

 accurate vision or inspection. 



Focusing and adjusting for cover-glass thickness can be 

 learned by experience only. As is well known, however, both 

 arts are of the most vital importance. In this paper I shall 

 not endeavor to give any advice on these two points except 

 merely to mention a little matter that I have never seen 

 elsewhere stated and which has been of great service to me. 

 In my experience the microscope is always horizontal ; 

 this is the convenient position. 



One day when working at the instrument I discovered 

 that when I placed my fingers on the milled head of the fine 

 adjustment screw, there ensued an alteration of the focus 

 although the head had not been turned. Further investi- 

 gation brought out the fact that the alteration was due to a 

 springing of the arm induced by a downward pressure on 

 the milled head, and that when the finger was removed the 

 object came again into perfect focus. I also found that a slight pressure 

 upward caused the object to pass out of focus in the opposite direction. 



This proved to be an exceedingly delicate test of the correctness of the focus. 

 If perfectly focused the error produced by this slight pressure is equal in both 

 directions ; but if not perfectly focused the error will be more evident in one 

 direction than in the other. This apparently commonplace and trifling matter 

 is well worth the attention of anyone who attempts the photography of these 

 very delicate and difficult forms. 



The time of exposure will, of course, vary according to conditions. I use two 

 difTerent amplifications, 660 diameters and lolJO diameters. All my photographs 

 are made at 6(30 diameters unless the objects are very small or are adorned with 

 very fine stria;. When the forms are large and marked with fine striae two 

 photographs are taken ; one to show simply the outline, and the other at the 



Fig. 2. 



