41 



be large enough for the cone of light entering through it to fill one-third 

 of the central bright portion of the objective; to ascertain whether this 

 is so or not one looks into the microscope tube when the eyepiece is 

 in with a lens such as is often used for focusing on the ground glass; 

 this must be done with every objective used with the Abbe and the exact 

 point to which the diaphragm is opened should be observed on its gradu- 

 ated scale and recorded; if this is not done, and guesses are relied on, 

 hit and miss (mostly miss) results need only be expected. Too wide a 

 diaphragm will drown the details in light; too small a diaphragm will 

 surround all details with diffraction halos that will gain in ugliness as 

 one learns them better. 



(d) Ray Filters. 



The various colors of white light have differing values for optical and 

 photo-chemical purposes; they do not focus after being refracted at the 

 same place. When the apparatus is so adjusted that the red, orange and 

 yellow rays which mainly affect the eye are in average focus on the 

 ground glass, the blue and violet rays, which mainly affect the sensitive 

 plate, will be in focus enough nearer the object to spoil the picture. 

 One good way to overcome this difficulty is to use a color screen, which 

 cuts out the red and orange rays and at the same time thn blue, indigo and 

 violet rays at the other end of the spectrum, leaving the yellow-green 

 waves of approximately the same wave-length to affect both the eye and 

 the plate; without this precaution a good photomicrograph can not be 

 made with daylight or the electric arc; such a color screen is best produced 

 by placing in the path of the light a glass trough with parallel sides and 

 about three-sixteenths of an inch thick, filled with the following solution: 



160 grams of dry, pure copper nitrate. 

 14 grams of pure chromic acid. 

 125 cc. of distilled water. 



This is Zettnow's filter. We have found great advantage, especially in 

 photographing preparations stained with saffrannin or fuchsine. in adding 

 a second trough filled with a dilute solution of Loeffler's methylene blue. 



