376 HOW TO SEE WITH THE MICROSCOPE. 



said I, " is this thickened edge not the fault of the objective, 

 and suppose that we examine the specimen with another glass?" 



Removing the London lens, I substituted a wide-angled l-6th 

 of plus 180 of aperture, and on adjusting carefully to the thick- 

 ness of covering glass, I soon had a splendid view of the pre- 

 paration, by far finer than that shown by the London glass ; 

 and as I had supposed would be the case, I now had a nice 

 clean edge in the place of the thickened one before mentioned. 

 The superiority of the definition over that of the l-4th was 

 apparent; and tht capacity of the 1 -6th to bear the 1 -4th inch 

 solid eye-piece and amplifier thus carrying the amplification 

 up to nearly 4,000 diameters was in a few moments demonstra- 

 ted, but the Nasmyth's membrane was not to be seen under any 

 amplification. 



At this time it occurred to me that I might possibly make a 

 membrane as described by the books, and as shown by the Lon- 

 don lens. Turning the collar of the l-6th so as to throw the 

 glass considerably out of adjustment, and again focussing the 

 object. I was much amused to find that I had the identical 

 membrane, as shown by the imported glass, and also, that as 

 tht collar was moved, the membrane could be made thick or 

 thin to suit ! 



The result of this little episode was simply this my friend 

 now owns and uses on histological work an American made l-6th 

 of 180 of aperture. 



I have related, as briefly as possible, this little incident, and 

 exactly as it occurred. I desire to say, however, that I do not 

 deny the existence of " Nasmyth's membrane," described in the 

 various works on physiology and histology. Nevertheless can- 

 dor compels me to state that I have in times past made diligent 

 search, and have failed to recognize a structure which is taught 

 even in our elementary school books, as if it were a common- 

 place thing I 



Months ago it occurred to me that the membrane referred to 

 might only be found on teeth of the first dentition, and obvi- 

 ously would be found wanting on the tooth of an adult. This 

 idea sent me to the dentist's office ; such specimens were pro- 

 cured, but the " Nasmyth " still defeated my search therefor. 



