No. 3.] THE EGG OF ALLOLOBOPHORA FOETIDA. 143 



times, we found that the variation of 2^^ almost unfailingly 

 caught the desired focus. 



As two or three photographs can be taken in as many 

 minutes, we generally take three of each preparation, one at 

 the tested focus, one 2^^ above, and one 2V below this point, in 

 case any difference in temperature, thickness of the cedar oil, 

 or any other unforeseen factor should affect the focus. The few 

 minutes required to develop these extra negatives is time well 

 spent, for occasionally the focus above or below the tested one 

 proves the best. This variation in focus can be just as readily 

 settled for any magnification, and it does away entirely with the 

 hopeless effort of attempting to focus fine details on the ground 

 glass. We have tested the method with different magnifications 

 and different lenses, and find it works admirably in all combina- 

 tions, but it seems unnecessary to give figures for the different 

 tests, as the variation is undoubtedly a point that must be 

 settled for each microscope. 



A method of work that can aid the biologist in seizing accu- 

 rately and rapidly any points of interest his material offers, and 

 enables him to retain them in a form convenient for comparative 

 study, must certainly be of great value in the laboratory. 

 Photomicrography appears to us to fill just such a need. A 

 dozen photographs of a variety of features can be taken in the 

 time required to reproduce any one of them by a careful draw- 

 ing. The printed photographs can be kept in a form service- 

 able for frequent reference, and the impression first made by 

 the preparation not allowed to fade. Possibly a photograph is 

 less intelligible than a simplified sketch to any one unfamiliar 

 with the preparation, but cannot this be said of the preparation 

 itself .'' We have collected over two hundred sketches and as 

 many photographs, illustrating features in our sections we 

 wished to preserve for comparative study. Of the relative 

 values of these two methods there can be no question, in every 

 case the photographs proving to be the more valuable aid in 

 recalling the preparations. We are not pleading to replace the 

 sketch with the photograph, but we would argue for the use of 

 both, letting the photograph speak for the preparation and the 



