In case you should at any time require it, I can specify more in 



I have given all the observations in order that by comparing them, 

 you may see, in the case of the coarse diatoms which are very 

 easy to measure, the amount of variation in coarseness on the 

 same frustule ; and, in case of the finer, which more easily admit 

 of some error in measurement, the degree of accuracy shown by 

 the accordance of different observations. 



It is not till I shall have measured several copies of Moeller's 

 probe-platte, that it will be proper or worth while to make any de- 

 tailed comparisons of the results. It may, however, be remarked 

 that the two specimens of T. favus differ as much as nineteen or 

 twenty per cent. Two vary from fifteen to twenty per cent ; two 

 vary from ten to fifteen per cent ; six vary from five to ten per 

 cent; ten vary less than five per cent; as compared with my 

 probe-platte. The average variation between the measurements 

 of the corresponding diatoms of the two plates is six and nine- 

 tenths per cent ; the average of all the variations shows your 

 platte to have its frustrates five per cent, finer than mine. The 

 samples of Amphipleura pellucida on the two plates agree within 

 three and two-tenths per cent.— Very truly yours, Edward W. 

 Moklet, Hudson, 0., April 10, 1875.' 



American Association. — A full representation of those inter- 

 ested in the microscope is especially desirable at the Detroit meet- 

 ing of the A. A. A. S., commencing on the 11th of next August, 

 as it is desired to take steps toward the organization of a Micro- 

 scopical Society, either as a separate society or club, or as a sub- 

 section of the large Association. There is a very general desire 

 for a society of American microscopists, and it is believed that 

 such a society can obtain general attendance from the whole 



