382 HOW TO SEE WITH THE MICROSCOPE. 



the " run" of the fine adjustment is known, the plan is a con- 

 venient one, and will give measures tolerably correct ; but for 

 the purpose of adjusting an object-glass it is obviously fanlty, 

 and would in almost every instance defeat such wide angled 

 lenses as I am in the habit of using. Dr. Beale's instructions 

 nsjto the adjustment of wide-angled lenses must be taken "cum 

 grano salis," aud will indeed be more honored in the breach 

 than in the observance. 



It is with pleasure that I now turn to page 217, and read as 

 follows : " Besides extreme minuteness in mere size, extreme 

 tenuity or transparancy may interfere with the definition of an 

 object. Now the greatest difference is observed in object- 

 glasses in this particular. The best object-glasses will define 

 clearly and accurately mark the words," clearly and accurately" 

 bodies which, from their transparency, are quite invisible 

 under objectives only slightly inferior to the first." 



Now the question arises at once, Where shall we find these 

 best object-glasses, which define so clearly and so accurately 

 such delicate and transparent objects? Referring again to 

 page 7, Dr. Beale responds thus : " Glasses with a high angle 

 of aperture admit much light, and define many objects of an 

 exceedingly delicate nature, which look confused when exam- 

 ined by ordinary powers." 



The Doctor having thus squarely settled the question, most 

 kindly goes on to tell us where we can get these wide-angled 

 glasses, and I read right along, as before quoted : " Mr. Boss 

 has lately made glasses having an angle of 170, which are valu- 

 able for investigations upon very many delicate and thin struc- 

 tures," etc., etc. 



It is especially a part of my purpose in appearing before you 

 this evening to show you " clearly and accurately," and by the 

 aid of glasses of American manufacture, which for width of 

 aperture have never been excelled, some of these extremely thin 

 .and transparent objects, and thus prove to you the truth of Dr. 

 Beale's assertion ; and relying on the correctness of the old 

 adage that " seeing is believing," I can hardly fail in the 

 -demonstration of the accuracy of Dr. Beale's position. 



Pardon a momentary digression, and allow me to turn to page 



