854 Transactions of the Society. 



that it would not be found available for the high powers I desired 

 to use. Amplifications of from one to two thousand diameters were 

 what I wanted to have at command, and until quite lately it did 

 not seem likely that this could be secured in photography by 

 lamplight. 



During the preparation of the articles on diatom structure above 

 referred to, I determined, with some hesitation, to test the use- 

 fulness of this method of illustration. Beginning in April, I have 

 made between fifty and sixty negatives of what I have called a 

 "broken shell series," and from which the accompanying set is 

 selected. 



The apparatus I use is very simple. It consists of Walmsley's 

 photo-micrographic camera with cone of pajner machS attached, and 

 a common coal-oil lamp with broad flat wick an inch and a half 

 wide. In selecting a lamp I chose one having a strong draught 

 and good combustion giving an intense white flame. In using it 

 the edge of the flame is turned to the Microscope, as in the resolu- 

 tion of difficult tests. To obtain the desired amplification, even 

 with a 1/15 in. objective, the full extension of the camera bellows 

 is necessary, or the use of an amplifier in the body tube. 

 Without pretending to be sure that my method is the best, I will 

 still say that I have thus far got the best results by using the No. 1 

 eye-piece in the Microscope, and no other amplifier. It seems to 

 me that after correcting the objective with care so as to present 

 the best results to the eye directly, the satisfactoriness of image 

 which is thus produced is best kept by using both objective and 

 eye-piece in photographing, precisely as in ordinary observation, 

 and with the same length of tube; changing nothing but the 

 fine adjustment to correct the focus for the position of the camera 

 screen. Such, at least, is the conclusion I have tentatively 

 reached. 



The thing I have specially aimed at has been to correct the 

 objective by the collar with the utmost care to procure sharp 

 definition of the broken edges of the valves, and to reduce the 

 diffraction as much as possible, also, by this means and by the 

 manipulation of the light. After patient experimenting to secure 

 this, I place the tube in a horizontal position and attach it to the 

 camera with as little change of conditions as may be. I use an 

 achromatic condenser which is a slight modification of a Kellner 

 eye-piece, with violet blue modifier and a variety of movable dia- 

 phragms and stops at the back. These were not specially provided 

 for photography, but being such as I am in the habit of using in 

 actual investigations, I have, on the principle before stated, 

 continued their use with the camera. 



With the exception of one or two negatives, my photographs 



