The Microscope. 143 



Whilst many studied the protoplasm and the cell mem- 

 brane, a few also gave their attention to the study of the nu- 

 cleus, but they met with little success. It had been stated for a 

 long time, that the living nucleus appears within the protoplasm 

 as a clear spot, more or less granular in structure and contain- 

 ing an aqueous fluid. Schleiden (1838) had described and illus- 

 trated some nuclei with contracted granules appearing to be 

 arranged in a certain order^. 



Nffigeli (1844) and Unger (1846) had observed that the nu- 

 cleus in many cases is also surrounded by a membrane. In the 

 nucleus, microscopists had reached the limit of investigation, 

 for want of perfection in their instruments. During the subse- 

 quent twenty years no advance was made in the study of the 

 nucleus. To-day one thing was affirmed by one to be denied 

 "the next day by another, and as to the nature of the nucleolus, 

 the wildest statements were made. Finally, the study of the 

 nucleolus was totally abandoned for want of suitable instru- 

 ments. 



In conclusion let us call attention to a radical mistake made 

 almost by all investigators in their study of the nucleus. As a 

 rule the nuclei of the eggs were chosen as the subject of investi- 

 grtion, but these nuclei are the worst kind that could have been 

 selected. True, they are generally of large size, but they are far 

 from being typical and normal. 



THE MEASUREMENT OF APERTURE AND WORKING 



DISTANCE. 



GEORGE E. BLACKHAM, M. D., F. R. M. S 



THE angular aperture of an objective is properly the angular 

 value of the difference in the path of the extreme rays of 

 the widest pencil of rays which can be utilized by it for the pro- 

 duction of a well defined image. Imperfectly corrected lenses 

 may transmit rays more oblique than can be brought to a com- 

 mon focus with the less oblique central rays, but as these rays 

 do not contribute to the formation of a well defined image, serv- 

 ing on the contrary, only to confuse and distort it, they have no 

 claim to be included in the available aperture of the lens. 



It would seem hardly necessary to argue that in order to as- 



3 Evidently he had mistaken the convolutions, or protuberances of the nucleolus. 



