No. 3] THE EGG OF ALLOLOBOPHORA FOETID A. 141 



lenses 16-2 mm. immer., 140 aperture, Zeiss compensating and 

 projection oculars. We have not used a magnification beyond 

 one thousand, finding this will reproduce details that can be 

 clearly seen only with a 2 mm. immer. and ocular 8. 



With a 2 mm. immer. projection ocular 4, diaphragm of ocu- 

 lar at o, and the longest draw, the magnification attained is 

 nearly one thousand [about 960]. When the diaphragm of 

 projection ocular 4 is adjusted to 10, the magnification is 

 much less ; e.g., a draw giving a magnification of 520, with the 

 ocular adjusted at 10, will give a magnification of 670, with 

 adjustment at o. We tested our magnification by measuring 

 the object with a Zeiss micrometer eye-piece, then taking the 

 measure of the photograph in microns, and dividing the latter 

 by the former. On account of the difference in magnification 

 dependent upon the adjustment of the projection ocular, we 

 found this the only accurate method. Light — clear daylight ; 

 sun shining, but not on mirror. Time — as near noon as 

 convenient. Exposure — as a rule, fifteen to thirty seconds for 

 sections.^ 



It gives us pleasure to express our indebtedness to Prof. 

 Henry Crew, of Northwestern University, for recommending 

 to us the following methods of developing and printing, and 

 for instruction in their use. Plates — Seed 27. Developer — 

 Metol. Printing paper — Kirkland's Lithium. 



We wish also to express our obligation to Mr. J. G. Hubbard 

 for our first lessons in developing and printing. 



The experiments of a year with photomicrography have con- 

 vinced us of its utility as a practical aid in cytological investi- 

 gation, and we hope in this paper to argue for its more general 

 adoption. The impossibility of photographing fine cytological 

 details, which can be readily illustrated by a drawing, has been 

 urged by Flemming (3) and others as the principal argument 

 against the use of the camera in cytological work, Wilson's 

 atlas (9) and Erlanger's photographs (2) serving to support 

 these objections. 



Those who have attempted to photograph cytological details 



1 For photography by daylight it is necessary to have a time shutter in the 

 camera. 



