190 



■THE AMERICAN MONTHLY 



[Nov 



Gates. He intends hereafter to use a heliostat instead 

 of an arc-lamp. 



In a recent letter to the editor of this journal he says: 

 "I enclose a picture of the new double microscope (frontis- 

 piece). You understand that it is extremely difficult to 

 keep the arc-lamp, the condensing lenses, the alum filter, 

 the parallelizing lenses, the two microscopes and the 



camera in alignment, and that when once focussed to se- 

 cure a good definition that it is extremely difiicult to ad- 

 just the camera so as not to throw the image out of focus. 

 "Fig. 1, is a photomicrograph of Pleurosigma angu- 

 latum made with a one-sixth inch objective and a one 

 inch ocular in the first microscope and 14 millimeter ob- 

 jective and a one inch ocular in the second microscope. 

 The definition in this particular photograph is bad but as 



