651 



XVI. — Systematic Exposure with Transmitted Light in Photo- 

 micrography. 



By Alfred Letherby. 



(Read October 16, 1907.) 



When the Microscope has l)een set up and the image focused on 

 the screen of the camera, a critical moment has arrived. The 

 resulting negative, be it good, indifferent, or useless, will be 

 dependent upon the exposure given. In deciding what this shall 

 be, the operator is influenced by three governing factors : — 



1. The intensity of the illumination. 



2. The nature, colour, and density of the object. 



3. The speed of the plate. 



Watkins' speed card, or Welcome's exposure tables, provide a 

 sufficient guide for the last, but the first and second have to be 

 obtained from trial exposures under identical conditions ; or at the 

 best be an approximate estimate based on the appearance of the 

 image on the screen. 



If the operator is in the habit of working only on these limited 

 lines, his method cannot be regarded as a satisfactory one, and it 

 is either always necessary for him to set up his apparatus on the 

 lines of former work, or court failure by a possibly mistaken 

 calculation. 



All photomicrographers will readily admit, it is desirable to be 

 able to set up the object with such lenses, with such intensity of 

 light, and with such magnification, as is most suited to display the 

 object at its best, without first referring to former attempts, and to 

 expose, without estimating by appearance the brilliancy, or want 

 of brilliancy, of the image on the screen. 



In order to achieve this, the appended table has been devised, 

 and although it does not pretend to mathematical accuracy, it will 

 be found on trial to fulfil the condition for all practical purposes. 



The table itself cannot be used as it is intended, without a few 

 satisfactory trial exposures, for the purpose of discovering one of 

 the factors involved in its use. These must be made by each 

 operator, and this is al)Solutely necessary, as no two individuals 

 are likely to have identical factors for the production of the 

 photograph. 



The factor with the widest difference would probably be the 

 light, and the next widest the focus of the condenser. Now it is 

 evident no two operators will have command of the same illu- 



