1899] MICROS CO PICAL JOURNAL 153 



cover to the cell, both fast and hermetically. It is best to 

 seal at two opposite points first, so as to fix the cover at 

 once and then fill up the intervals. Objectives up to the 

 Zeiss D (low angle \' r ) are available. 



Besides the other advantages of this system, we may 

 note that the animals are evidently much more at home 

 in this prison than in any of the ordinary devices. The 

 shyest tubicolous Rotifers and Polyzoa come forth readi- 

 ly, stay well exposed, stand a good deal of motion with- 

 out shrinking back, and when they do withdraw return 

 into view with theleastpossible delay ; and free swimming 

 animals may be so confined by limiting the size of the 

 drop as to keep them in the field under relatively high 

 magnification. I am free to admit that the method is 

 more troublesome than putting between two pieces of 

 glass, a thick one and a thin one, but with the paraffin 

 cell, once done, all is secure for the evening, the only 

 possible dangers being those that all microscopic mounts 

 are liable to. — III. Am. Mic. 



Uniformity and Accuracy. 



M. I. CROSS. 



To the working microscopist it is a great advantage, 

 amounting almost to a necessity, that the objectives pro- 

 duced by the different makers shall interchange in the 

 nosepiece of his microscope ; but many of us have found 

 that it has been impossible to screw home some objectives 

 we have obtained until the thread has been eased, and 

 have at once cast blame on the manufacturer. He, how- 

 ever, has not hitherto been altogether at fault, for no 

 doubt he has worked to his standard gauges supplied by 

 the Royal Microscopical Society many years ago, and is 

 entitled to aver that his lenses are screwed to the "uni- 

 versal" thread and that others must be wrong. 



Now this "universal" thread has for years been a mis- 

 nomer, the gauges issued by the Royal Microscop. society 



