1889.] NEW-YORK MICROSCOPICAL SOCIETY. 113 



time. Then they cut their own food. But, in time, their 

 mandibles become worn and dull. When metamorphosis takes 

 place, the new form has a pair of new and sharp mandibles, and 

 the wood cutting can be renewed. The nymphee also dull 

 their mandibles. The imago has at first a new pair of man- 

 dibles, and can cut hard wood. When these mandibles become 

 dull, then a supply of food must be cut for the imago by those 

 having sharper mandibles. 



PROCEEDINGS. 



Meeting of May 3D, 1S89. 



The meeting was held in the Lecture-Room of the Chemical 

 Department of the School of Mines, Columbia College. 



The President, Mr. Charles F. Cox, in the chair. 



Thirty-nine persons present. 



Prof. Edw. G. Love addressed the Society on " Photomi- 

 crography." Prof. Love gave a rapid sketch of the history of 

 Photography from its first beginnings, making special reference 

 to its use in producing representations of microscopic objects. 



The description of the needed apparatus took the following 

 form : — 



1. Sources of illumination, in the order of their value. TJie 

 methods for obtaining monochromatic light, by means of the 

 prism, or the ammonio-sulphate of copper solution, were here 

 explained. 



2. The Microscope. Regarding the stand, the speaker 

 explained that a short tube, which must be lined with black 

 velvet to prevent all internal reflection, and be used without an 

 eye-piece, would give the best results ; that a mechanical stage 

 was very desirable, as well as a sub-stage, for holding an achro- 

 matic condenser, when using high powers. 



3. The Focusing Screen. A ground-glass screen would answer, 

 when sun-light was used with low powers. But some finer sur- 

 face would be found necessary with other sources of illumination, 

 and with the use of moderate to high powers. Among the sub- 

 stitutes employed, a dry plate, which had been slightly exposed, 

 and then bleached after being developed and fixed, would afford 

 a very good surface. But the best screen, unquestionably, was 



