1869.] 361 [Greenleaf. 



croscopes for short-stand instruments, and even here T was not sur- 

 prised to see the Professor of Pathological Anatomy using a short- 

 stand French microscope. 



Doubtless every observer will handle his own instrument to greater 

 advantage, but for certain pui'poses particular constructions are 

 preferable ; and indeed I know of do work that would actually 

 require a high-stand microscope. I am the better able to judge, 

 having examined microscopes of this kind in Germany and England, 

 especially those of Frauenhofer, Ross, Smith, Amici and others, 

 It may be interesting to mention that Nobert's instruments are not 

 considered superior. I have examined a first-class microscope with 

 an objective fitted for correction, and calculated by him to have a 

 power of 500 diameters. The marked yellow light in the Nobert 

 microscopes is very trying to the eyes. The mechanical work is 

 good but not remarkable. A kind of screw for fine motion used 

 by him is perhajis unknown. A long, strong, steel screw is used ; 

 the upper half of the thread of which is turned in the opposite direc- 

 tion from that on the under half, and the two halves differ somewhat 

 in size. By this arrangement the motion of the screw moves the in- 

 strument only as far as the difference in the fineness of the two halves, 

 and with a strong screw a very fine motion is obtained, and " dead 

 point" is impossible. 



The Trichina spiralis has singularly forwarded the manufacture of 

 microscopes. Every physician and many other persons engaged in 

 examining pork, tried to obtain a microscope as soon as possible. At 

 first the manufacturers could not possibly meet the demand. Conse- 

 quently the manufacture of these instruments has everywhere increased, 

 and one can get a very good French or German student's microscope, 

 amplifying 250 to 300 times for twenty-five dollars. I have seen in- 

 struments with a power of 150 to 200 times for twenty dollars or 

 even less. The increasing number of instruments has been very ad- 

 vantageous to science, and I hope that the calamity of trichina, even 

 now fearfully prevalent in Europe, will be compensated by a marked 

 progress in science. 



Mr. R. C. Greenleaf offered the following remarks on the 

 double plate of Aulacodiscus oreganus. 



Mr. Charles Stodder, in several communications to the Section, has 

 called attention to the double plate of the various disk forms among 

 the diatoms. A few days since. Mi*. E. Samuels, who is again giving 



