NEGATIVE MAKING 67 



from either side will suffice to move the ad- 

 justment screw in the most delicate manner. 

 It is not worth while to repeat the processes 

 already shown, but may not be amiss to fol- 

 low the making of the two negatives I have 

 chosen for illustrations of medium power 

 work. 



In order that my friends, the diatomaniacs, 

 may not be neglected, I have selected one of 

 the beautiful discoid forms, Aulacodiscus Ore- 

 ganus, for our first attempt at somewhat higher 

 magnification than the earlier ones. This 

 diatom is almost perfectly flat in the center of 

 its disc, but the margin comprises a series of 

 processes considerably elevated, which make it 

 quite impossible to get the whole surface into 

 one perfect focus with an objective of even 

 moderate power. We therefore have to com- 

 promise on the focusing, in order to obtain a 

 passable whole, and Ross' students' i-inch seem- 

 ing to combine a low angle with excellent resolv- 

 ing powers, I have chosen it as the objective. 

 A coal oil lamp, in the absence of something 

 better, must suffice for the radiant, and as you 

 will see, I am using the edge of flame in con- 

 nection with a bull's-eye condenser, shown in 

 Figures 4, 5 and 6. An Abbe chromatic con- 

 denser, with iris diaphragm, is fitted to the 



