and Laboratory Methods. 



1729 



It is only in photomicrographs of a thousand diameters and upwards that 

 the real difficulties of focusing begin. It is a good plan when an immersion 

 exposure has been made to immediately replace the ground glass to see whether 

 or not the focus has by any accident changed during the exposure. Real prog- 

 ress can also be made by faithfully comparing every negative that shows defect- 

 ive focus with the image on the ground glass. 



I would especially recommend as a valuable study in sharpness of the image 

 an object which has clear detail in several different planes, as for instance a 

 Golgi nerve cell or a kariokinetic figure with iron-haematoxylin stain ; focus on 

 some clear detail in a middle plane ; having made an exposure compare care- 

 fully the negative with the image on the ground glass ; the relationship between 



Fig. 3. 



Fig. 4. 



the negative focus and the ground glass appearance can in this way be correctly 

 mastered. The beginner should stick to this practice until he can pick out his 

 detail in such a slide and correctly reproduce it in his negative. Figs. 1 and 

 2 show such a study; the centrosome in one cell of Fig. 1 is sharp and in the 

 other cell of Fig. 2. 



Every one who attempts to make photomicrographs meets the difficuhy of 

 focusing; with a bright light, a low power, and a means of controlling the 

 adjustment screws, this difficulty vanishes; the Misses Foote and Strobell of 

 New York City, who have a seaside laboratory at Woods Hole, have met this 

 difficulty in powers up to a thousand in an entirely novel way. Their method is 

 simple, and the necessary apparatus in addition to a good microscope need not 

 cost more than five dollars. The microscope is put in an upright position with 



