166 



THE AMERICAN MONTHLY 



[September, 



long time — until it has acquired a 

 knife-edge parallel with the line of 

 motion, in which only a single crystal 

 does the cutting. For fine lines the 

 point improves by use. * * * * 



If Mr. Fasoldt's claim is reduced 

 by about one half, I am by no means 

 sure, but it may be realized. But 

 what evidence have we that it is pos- 

 sible to see lines of this degree of 

 fineness ? 



In the matter of limit of resolution, 

 it must be admitted that little or no 

 progress has been made since the 

 resolution of Nobert's 19th band. * 

 * * I cannot learn that any one has 

 yet succeeded in photographing a 

 Fasoldt plate as high as 100,000 to 

 the inch. With due respect for the 

 honest belief of several microscopists 

 who claim to have resolved Fasoldt's 

 bands as high as 152,000 to the inch, 

 I yet hold to the opinion that in 

 no case has the resolution been proven. 

 There is only one test which is abso- 

 lutely decisive — that of ruling a 

 definite number of lines in a band, 

 and keeping the number secret until 

 the microscopist can give the correct 

 count, not merely in one instance, 

 but in several. * * * The ap- 

 parent number of lines in coarse 

 bands can be varied. Can any one 

 offer a reason why there should not 

 be a similar variation in fine bands ? 

 In certain cases there is no certainty 

 that the lines are actually cut in the 

 glass. There is often an appearance 

 of perfect lines, but they disappear 

 when the surface is rubbed. Mr. 

 Hitchcock, in a recent number of his 

 Journal, has made the claim that re- 

 solution has, to a certain extent, 

 ceased to be a test of the quality of 

 an objective. I presume this claim 

 will be found to have some founda- 

 tion in fact. For the last ten years 

 we have only the assertion of resolu- 

 tion, but not the proof. It is time 

 the proof should accompany the as- 

 sertion. I insist that the simple 

 vision does not afford the required 

 proof. * * * Before we can 

 safely assert that observation has 



gone beyond theory, we inust be pre- 

 pared to offer evidence which can be 

 placed upon record, discussed, and 

 impartially weighed. * * * Let 

 us have a test which will forever set 

 at rest this vexed question of resolu- 

 tion. * * * Let Mr. Fasoldt rule 

 three plates, under as nearly the same 

 conditions as possible except in the 

 number of lines in the different bands 

 of each plate. Let him label each 

 plate, and accompany it with a full 

 description of the number of lines in 

 each band. Let these plates be sent 

 to any gentleman in whom the great 

 body of microscopists have confi- 

 dence as eminently qualified to con- 

 duct an investigation of this kind. 

 Let whoever receives the plates re- 

 move the labels of Mr. Fasoldt, and put 

 in their place labels whose significance 

 is known only to him. Then let those 

 gentlemen who think they have re- 

 solved 152,000 lines to the inch 

 make their count of the lines in each 

 band, and send in their reports. Let 

 the plates also be photographed and 

 let the number of lines be counted. 

 Then let the results of these investi- 

 gations be published. If all sub- 

 stantially agree in the count, this will 

 end discussion. 



The paper concluded with an ac- 

 count of some experiments with very 

 fine single lines. Lines too fine to be 

 seen singly with the microscope can 

 be resolved if ruled close together in 

 bands. Single lines estimated to be 

 '6o,oi)'o of 3,n inch are readily seen by 

 the naked eye, and even lines f^^Voir 

 of an inch in width can be seen with- 

 out a microscope. A plate was shown 

 with the coarser lines ruled upon it, 

 and which were easily seen by every 

 one who tried to find them. 



The following is an abstract of a 

 paper read by Dr. Thomas Taylor : — 



THE HOUSE-FLY AS A CARRIER 

 OF CONTAGION. 



About eighteen months ago, while 

 dissecting the head of a common 

 house-fly, I observed a very minute, 

 snake-like animal, a species of anguil- 



