904 



TPIE AMERICAN MONTHLY 



[November, 



" test-objects " for English achro- 

 matics. At that time, our best work- 

 ers in biological investigation were 

 using — not the compound micro- 

 scope — but the Wollaston '* doublet," 

 which had been greatly improved by 

 its constructors ; and, for the highest 

 powers, a " triplet " made on the 

 same plan. It was with these that 

 my late friend Prof. Sharpey, carried 

 out those admirable researches on 

 ciliary movement, which early gained 

 for him a deserved reputation ; and 

 that Mr. Slack executed his beautiful 

 dissections of vegetable tissues. 



It was at this time — about half a 

 century ago — that the achromatic 

 combinations made by the French 

 opticians, having a magnifying power 

 considerably greater than that of 

 Tulley, were first introduced into 

 London. The power of these com- 

 binations was gained by screwing, one 

 upon another, three, or even four 

 pairs, each consisting of a double- 

 convex of crown, and a plano-con- 

 cave of flint ; such as are now sup- 

 plied with the cheapest French mi- 

 croscopes, — the back pair being of 

 lower power, so as, when used alone, 

 to give about the same amplification 

 as Tulley 's inch. This plan gave re- 

 sults for what were then accounted 

 high-powers, which caused it to be 

 forthwith adopted by the London 

 makers Ross, and Powell, who had 

 been previously constructing Wollas- 

 ton doublets and triplets ; and they 

 soon produced objectives of one- 

 fourth or one-sixth inch focus, quite 

 equal to those of French makers, and 

 low powers much superior to theirs. 



The next great improvement was 

 the result of the theoretical investi- 

 gations of Mr. Lister, the father of 

 Prof. Lister (of " anaesthetic sur- 

 gery " fame), and uncle of the Messrs. 

 Beck. Mr. Lister (whom I well 

 knew), was a gentleman of consider- 

 able scientific attainments ; who de- 

 voted a large amount of time and 

 money to the improvement of the 

 achromatic microscope, and was 

 himself a worker of no mean ability 



in marine zoology. By the applica- 

 tion of mathematical analysis to the 

 designing of microscope objectives^ 

 he showed that by giving to each 

 pair of the triple combination a 

 special relation to the other pairs,, 

 the errors of the whole could be 

 much more completely eliminated 

 than they were by the existing 

 method ; and that not only a more 

 perfect correction, but a great en- 

 largement of the angle, could be thus 

 obtained. He endeavored to induce 

 Ross and Powell to begin afresh upon, 

 his principle ; but not finding them 

 disposed to do so, he took up James 

 Smith, then a mere workman, and 

 furnished him with the means of con- 

 structing objectives upon his own 

 designs. Smith soon produced a 

 "quarter " which was so superior ta 

 the best of Ross's and Powell's, that 

 the value of Mr. Lister's system was 

 at once universally recognized ; and 

 thenceforward, all makers have 

 worked upon it. In connection with. 

 James Smith, Mr. Lister also worked 

 out the plan of construction which 

 consisted in resting the "body," of 

 the microscope along its length up- 

 on a " limb," instead of supporting it 

 at its base alone ; and to his friend 

 Mr. Jackson, a surgeon who had a 

 mechanical turn, we are indebted for 

 the plan of swinging this limb be- 

 tween two side supports,^ instead of 

 attaching it by a " cradle-joint " on 

 the top of a single pillar, as in 

 the methods then followed by 

 Ross and Powell. Mr. Jackson's im- 

 provement originated in this way : — 

 Wishing to build a microscope for 

 himself, he had a difficulty about the 

 cradle-joint, which, before planing- 

 machines came into use, could only 

 be well constructed by a better filer 

 than he knew himself to be. Beings 

 on the other hand, a tolerable turn- 

 er, he saw that, by swinging his 

 limb on a horizontal axis, he could 

 rest the turned extremities of this 

 upon two side pillars ; and thus 

 was originated the " Jackson-Lister 

 model," which was first adopted by 



