MEDIUM-POWER PHOTO-MICROGEAPHY 127 



Theoretically the time of exposure in cases like this increases in the same ratio as 

 the squares of the diameters, or directly as the variation of the areas, i.e., if twice the 

 diameter (four times the area), it will be as i to 4. Put another way, supposing the 

 first picture is magnified 25 diameters, and the second 50 diameters, the exposure 

 varies as 25^:50', or as 625:2500, which is as 1:4. If then the first negative 

 was found to require two seconds' exposure, the second one should receive eight 

 seconds. 



This law holds good as a general guide, but it is not always found to be practically 

 correct, owing to the presence of a source of error hitherto not mentioned, and one not 

 so easy to intelligibly explain. When a specimen is photographed in its entirety any 

 small portions of it more dense than the rest become lost and unnoticed in the photo- 

 graph when taken as a whole ; by which is meant the lack of sufficient exposure ni 

 these minute parts of the specimen is unobserved. But when taking a photo-micro- 

 graph at a higher scale of magnification it is evident if we get one of these very places 

 to photograph, the law will not apparently serve us correctly, furnishing us very likely 

 with too short an exposure. But it is not the law that is at fault ; on the contrary, 

 it is really quite correct, for the very portion of the specimen which we have said 

 escaped the proper exposure before, now shows how clearly it was under-exposed. But 

 this may not strike the observer until after a little consideration. Anyhow, the law 

 furnishes us with an approximation if we have taken a photograph with a low power 

 first as described. But what if we have not, and are asked to take a photo of a tissue 

 at 300 diameters without any previous experience ? Speaking only of the limelight 

 mixed jet, it is a good rule to give an exposure (300 diameters being the magnifica- 

 tion) of about twenty seconds for the first plate and sixty for the second. It will be 

 evident then on which side of scale we shall have to proceed. If over-exposed, such 

 is the latitude of the Edwards plate that we may most probably save the picture. 



When using the 1 inch, the Davis diaphragm must be touched with a lighter hand 

 than before, and focussing becomes more of an art, and the head should be withdrawn 

 from the ground glass about ten inches, whilst a good dark focussing cloth excludes 

 all extraneous light. If any minute details are desired the coarse ground glass might 

 be removed for the finer one, after the photographer has made sure the light is equal 

 over the plate, and then if necessary the finest of all can be substituted, a Dallmeyer 

 or other focusser being applied while the operator turns his focussing screw gently to 

 and fro till he gets the best results. Great care must be exercised not to shake the 

 apparatus, as if we are using a six projection eye-piece, a |- inch objective and a 

 twenty-inch extension of our camera, which in our case is done by adding the auxiliary 



