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telescope was perfected, before similar improvements were applied 

 to the microscope. Indeed, notwithstanding the high mathematical 

 genius of some, and the great practical skill of others, who attended 

 to this subject, it was not till after the publication of Joseph 

 Jackson Lyster's paper in the Philosophical Transactions for the 

 year 1830, that the microscopist received with certainty a really 

 efficient achromatic object-glass. 



"Immediately after the publication of this important paper, 

 the attention of our best opticians was directed to the microscope, 

 and that race of improvement began which has led to such truly 

 marvellous results. 



" To be able to use a divergent pencil, of from 20° to 30°, was 

 then thought to be a great triumph, but by successive steps our ar- 

 tists have now, for deep powers, mastered pencils far exceeding 100°. 

 I have one by Nachet of Paris admitting 123°. I see Ross has ac- 

 complished an object-glass equivalent to one-fifth of an inch fo- 

 cus of 135° aperture; he has made several of one-twelfth focus 

 amounting to 145°. Smith and Beck of London make objectives 

 of very large angle ; and in a recent number of Silliman's Journal it 

 is announced that that truly worthy transatlantic competitor in this 

 field, Spencer, has completed an object-glass of about one-twelfth 

 inch focus, well utilizing a pencil of no less than 175° ! Perhaps 

 there has not been achieved any greater triumph of human skill, whe- 

 ther in reducing to practice the laws of light, or in moulding them 

 to the further development of nature, than these object-glasses. 



" But though so much has been accomplished, in the construc- 

 tion of the instrument, there probably remains much, not less im- 

 portant, respecting the arrangement of its illumination, as yet undis- 

 covered ; and as I believe every new fact in this department is valu- 

 able, I hope for the Academy's indulgence while submitting to it 

 some of my results. 



" For a very long period, much prior to the date of Mr. Lyster's 

 paper, I have been a practitioner with the microscope. The instru- 

 ment has always been a source of very high enjoyment to me, but 

 as my avocations have, I regret to say, prevented my being a conti- 

 nuous investigator, I have wished, and, as far as I could, sought 

 to be an improver. For the first few years I worked with the 



