THE MICROSCOPE. 17 



third of still higher importance. The microscopic examination of the 

 blood and other human organic matter, will, in all probability, more 

 than ever it has yet done, afford satisfactory and conclusive evidence 

 regarding the nature and seat of disease, than any hitherto appealed to ; 

 and will, of consequence, lead to similar certainty in the choice and 

 application of remedies. 



5. Soon after the year 1820, a series of experiments was begun in 

 France, by M. Selligues, which were followed up by Frauenhofer, at 

 Munich, by Amici, at Modena, by Chevalier, at Paris, and by the late 

 Mr. Tulley, of London. In 1824, the last-named artist, without 

 knowing \vbat had been done on the continent, made an attempt to 

 construct an achromatic object-glass for a compound microscope, and 

 produced one of 9-10ths of an inch focal length, composed of three 

 lenses, and transmitting a pencil of eighteen degrees. This was the 

 first that had been made in England. While these practical investi- 

 gations were in progress, the subject of achromatism engaged the 

 attention of some of the most profound mathematicians in England. 

 Sir John Herschel, Professor Airy, Professor Barlow, Mr. Coddington, 

 and others, contributed largely to the theoretical examination of the 

 subject ; and though the results of their labours were not immediately 

 applicable to the microscope, they essentially promoted its improve- 

 ment. Between this period and the year 1829, Mr. Joseph Jackson 

 Lister had directed his attention more particularly to this subject, and 

 he was led to the discovery of certain properties in achromatic com- 

 binations which had been before unobserved. A paper on the subject 

 was sent by him to, and published by, the Royal Society.* To the 

 practical optician the investigations and results of Mr. Lister proved 

 to be of the highest value ; and the progress of improvement was in 

 consequence extremely rapid, and since that period every year has 

 brought this instrument nearer to perfection. 



C. It would be foreign to the intentions of this work, to enter into 

 any lengthened details of the optical principles involved in the con- 

 struction of the microscope, or to trace out the various steps by which 

 the chromatic and spherical aberration, which rendered the early in- 

 struments useless, have now been overcome. To the optician and the 



* Philosophical Transactions, 1829. 



