SUPPLEMENT. 393 



properly used, for preliminary examinations, they are convenient 

 and useful, but for advanced work they should be abandoned, 

 and in favor of more inconvenient glasses of wider apertures. 



A wide-angled lens, incapable of receiving and utilizing cen- 

 tral beams, is a faulty and undesirable objective ; it is, in fact, 

 but half a glass. And on the other hand, a low-angled objec- 

 tive, incapable of receiving lateral beams, is. in my opinion, 

 equally undesirable, unless, indeed, when we are prepared to 

 sacrifice force of definition to convenience of handling. 



It must be admitted that there are functional and character- 

 istic differences in the performance of the two classes of objec- 

 tives; for instance, a wide-angled glass of relatively shorter 

 focal length will not see so far around an object as will one of 

 narrow aperture and longer focal distance.- This, as well as 

 other considerations which might be mentioned, offer no rebut- 

 ting testimony to the statement already advanced, that the 

 principal advantage presented by the low angles is their con- 

 venience in manipulation. 



In conclusion, I have to thank you for your kind attention to 

 the rambling remarks I have had the honor of presenting. Dur- 

 ing the few short hours that I have been in your city, the earth 

 has not had time to make one rotation ; but I have had a plenty 

 -of time to experience and enjoy a generous hospitality. I am 

 now ready to show you the promised objects, and shall be happy 

 to see what you have to offer. You will find me ready, willing 

 and ardent to be taught, and in microscope matters be assured 

 I am as absorbent as a sponge. 



A CHAPTER ON ELEMENTARY PHYSICS. 

 (Dedicated to Prof. Hitchcock.) 



Part First The Sun. u The luminous orb, the light of 

 which constitutes day ; the central body around which the earth 

 and planets revolve ; a celestial body which can be seen almost 

 any fair day through a piece of smoked glass." 



I beg my friend to read the above little monograph on ele- 

 mentary astronomy carefully. Assuming his compliance with 

 this request, I go on to say that just after sundown this evening, 

 my attention was called to his really kind notice of myself, as 



