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XII. — What did our Forefathers see in a Microscope ? 

 By Edward M. Nelson. 



(Read April 20th, 1910.) 



It is a question of some antiquarian interest what in pre-achro- 

 matic times did our forefathers see in a Microscope. In 1664 

 Samuel Pepys gave 51. 10s. for a Microscope, and said that " it 

 was a great price for a curious bauble/' but little did he think that 

 two and a quarter centuries later ten times that amount would be 

 paid for a Microscope-stand only, and five times as much for one 

 object-glass. Leaving, then, the " bauble " as something too 

 dreadful for contemplation, the question before us is what sort of 

 image would be seen in a Microscope of the highest type con- 

 structed before 1825 ? 



There were dioptric and catadioptric Microscopes in those days, 

 the dioptric being either single or double, or, as we should now 

 say, simple or compound, the catadioptric being always compound. 



A catadioptric Microscope would now be called a reflecting 

 Microscope, but formerly the term reflecting meant a Microscope 

 having attached to it a mirror solely for purposes of illumination, 

 and this, quite independently of its being either dioptric or cata- 

 dioptric. We have, therefore, to investigate three kinds of 

 instruments, viz., simple and compound dioptric, and compound 

 catadioptric Microscopes. 



Of old Microscopes, one of the best was that designed by 

 Dr. Smith ; it was catadioptric, and, consequently, achromatic. It 

 is fully described by its author in his " Complete System of 

 Optics," 1738 (pp. 87-97, plates 14 and 15 of the Eemarks), a 

 learned and excellent work which has been freely copied. The 

 description is given at length in old optical terms, so a translation 

 into those in modern use is necessary. Dr. Smith designed and de- 

 scribed four of these instruments (p. 91), of which that one numbered 

 IV. he had had made some years before the other three were com- 

 puted. He says, " I found it performed nearly as well in all 

 respects as the very best refracting Microscopes, and do not doubt 

 but it might have excelled them had it been more exactly 

 executed according to the proposed dimensions in the IV. column ; 

 where the angle of aberration, being but 6' 15", is above three 

 times less than the like angle in the best refracting Microscopes : 

 and since the angle of aberration of 39" in the III. column is near 

 ten times less than that of the IV., we have reason to expect that 

 a Microscope, exactly executed according to those dimensions, will 

 far excel any other yet invented." 



