142 FOOT AND STROBELL. [Vol. 1 1. 



realize the following technical difficulties. At a magnification 

 of one thousand, which is often necessary to bring out these 

 details, not enough light can be transmitted through our 

 thinnest sections to admit of focusing delicate structures on 

 the ground glass of the camera ; e.g., even with the aid of the 

 best focusing glass the small centrosome shown in Photo. 15 

 cannot possibly be seen on the ground glass. It is barely pos- 

 sible to see this detail through the microscope with a 2 mm. 

 immer. and ocular 8, and we were astonished to find it so dis- 

 tinctly reproduced in the photograph. The ring chromosomes 

 in Photo. 1 1 further illustrate this point. 



It is impossible, however, to focus such details on the ground 

 glass, and it has been our aim to devise some method of over- 

 coming this difficulty by discarding the ground glass as a factor 

 in focusing. 



Selecting a structure that could be clearly focused on the 

 ground glass at a magnification of one thousand (a sharply 

 stained nucleolus, for example), we first focused through the 

 microscope, making a note of the exact position of the pointer 

 on the face of the micrometer screw. We then slipped the 

 camera down, focusing the nucleolus on the ground glass. 

 The position of the pointer on the micrometer screw then indi- 

 cated exactly the difference between the two foci. In order to 

 facilitate the accurate measurement of this variation in focus 

 we marked off into twenty parts each of the twenty-five 

 divisions that are designated on the face of the micrometer 

 screw. The difference in focus proved to be about ttq of one 

 of the twenty-five divisions; e.g., with the pointer at the 5 mark 

 for the focus through the microscope, to get the focus on 

 the ground glass, turn the screw until the pointer indicates 

 4 and \\. 



We have tested the accuracy of this method by photograph- 

 ing at 960 diameters such a detail as the centrosome in Photo. 

 15. We took five photographs in as many minutes, keeping 

 all the factors unchanged except the focus. One photograph 

 was taken at what we calculated to be the correct focus; 

 i.e., i^Q above the focus through the microscope. Two were 

 taken above this point and two below. Trying this several 



