io THE MICROSCOPE. 



a pellucida in balsam (Moller's test plate) with entire clearness (as 

 to lines) by mathematically central sunlight and without accessory 

 apparatus of any kind ? I find that I can do it with great care with 

 my new Spencer ^ homog. immer. and one inch eye piece, giving 

 a power of only about 650." This was new to me, and I answered 

 that I had never seen a pellucida resolved by central light and was 

 not aware that it could be done with any lens yet made. March 2d 

 I received a second letter from Prof. F., in which he more fully 

 describes his methods as follows. (See his letter of March 2d, No. 1). 

 This letter I sent to Mr. Charles Stodder, the well-known micro- 

 scopist of Boston. Mr. Stodder replied that Dr. H. J. Detmers, of 

 the U. S. Department of Agriculture, had done the same thing with 

 a Tolles y 1 ^ homog. immer. made for him about 1878, and R. B. 

 Tolles, Esq., the veteran maker of our best lenses, sent a note saying 

 that "A. Y. Moore wrote a year or more ago that a ^ water immer. 

 duplex made for J. E. S. (Prof. Smith, of Cleveland, I suppose), 

 in 1874, did the same thing by central sunlight." Dr. Detmers, 

 after advising me of his success in accomplishing the same feat, 

 writes as follows. (See his letter of March 12, 1883, No. 2). But 

 it \s possible that either Dr. Detmers or Mr. Tolles may be in error. 

 I requested Prof. Forbes to try the effect of shutting off all possible 

 oblique rays of light by interposing two diaphragms between the 

 mirror and slide. He kindly complied, and under date of March 

 17th, writes as follows. (See his letter of March 17th, No. 3). This 

 seems to be thorough, and, so far as I know, is new to almost all 

 microscopists, and it may be of as much interest to others as it has 

 been to me, and it may involve some principle that can be made of 

 value in other investigations. 

 Chicago, March 23, 1883. 



Office of State Entomologist, )_ 

 Normal, 111., March 1, 1883. \ 



Friend Thomas — You are right in supposing that my objective 

 resolves A mphipleura pellucida by light which is strictly central, and 

 without the aid of any accessories whatever. The resolution is per- 

 fect all over the diatom, and so clear that any one can see it at a 

 glance. It is distinct, even with the camera lucida, and I have been 

 abje by that means to measure and count the striae, making between 

 95,000 and 100,000 to the inch. My first results were got by the 



