1899] 3iIOIlOSCOPiCAL JOURNAL. 155 



large size. Who can say how many different gauges are 

 nsed hy the various manufacturers for eyepiece sizes? 

 Yet tiie two sizes recommended would meet every want. 

 The two features named above are mucli-needed reforms, 

 and the only way in wliich they can be brought about is 

 for workers to stipulate for the society's sizes and accept 

 no other. 



When these alterations have been effected there are 

 plenty more matters that are worthy of attention, such 

 as makinu^ the different eyepieces of a series to work in 

 the same focal plane ; having all eyepieces engraved with 

 their initial magnifying powers rather than such absurd 

 letters as A, B, C, etc. Then it would not be a practical 

 impossibility for all the brass boxes, in which object 

 glasses are issued to be of uniform size or sizes, and for 

 object glasses to be corrected to one definite thickness of 

 cover-glass, which would in addition permit of the cor- 

 rection collars of all objectives being divided alike, so 

 that values should not have to be ascertained for each in- 

 dividual lens. If all these reforms were effected before 

 another Annual wore issued, many a microscopist would 

 have been enabled to do better and more accurate work 

 than hitherto. So much for uniformity, now for accuracy. 



On testing an apochromatic object glass stated by the 

 makers to have a numerical aperture of .95, some months 

 ago, I found that it was scarcely .87. I returned it, and 

 received an admission of the want of aperture, but not- 

 withstanding long and weary waiting, the firm — a very 

 distinguished one — has not been able to produce a lens 

 of the full aperture. This is a trivial matter compared 

 with some I have examined where tlie over-statement has 

 exceeded 20 per cent. It must be admitted that a sHirht 

 variation may occasionally occur, and in more than one in- 

 stance I have found an object glass to possess a larger 

 aperture than that attributed to it. It is, however, an un- 

 fortunate fact that in the competition which has arisen 



