34 THE GENERAL PRINCIPLES OF THE METHOD 



were lost owing to achromasia before the cooling tank 

 was employed. 



The photograph having thus been quickly taken, 

 the mirror may again be swung into position, the 

 camera pushed out of the way, and, having inserted 

 the working eye-piece, the examination of the specimen 

 may be proceeded with, or other fields explored. We 

 have taken a negative in fifty seconds with this appa- 

 ratus, and as many as five negatives have been taken 

 from different fields in a single specimen; but such 

 speed is not often necessary. 



All the photographs which illustrate this book have 

 been taken with the apparatus just described. It never 

 gives trouble, and has proved most useful in supplying 

 a means of recording the "specimens." It used to be 

 most annoying to see unique mitotic figures or other 

 interesting specimens slowly vanish before one's eyes 

 without being able to record them satisfactorily. In 

 fact, the best mitotic figure I have ever seen in a 

 lymphocyte was induced before we possessed a camera ; 

 and although thousands of figures have been seen 

 since then, I have never seen a picture comparable 

 to it. It w r as seen by Professor Harvey Gibson as 

 well as by myself. 



There is one other useful piece of apparatus which 

 requires mentioning, viz. the "revolving apparatus." 

 This is a simple clock-work contrivance which keeps 

 a long test-tube revolving on its long axis. The 

 test-tube is placed horizontally. The object of the 

 appliance is to keep the blood-cells constantly moving 

 in the "citrate solution," or other medium in which 



