286 HOW TO WORK 



1. A much thicker covering glass can be used than is possible 

 with the ordinary ^. 



2. The specimen is much more highly illuminated, the same 

 lamp, condenser, &c., being employed. The definition is, I think, 

 slightly better, but the difference is not sufficiently decided to enable 

 me to speak at all confidently upon this point. 



358. Of tlic One-fiftieth Objective. Since the last edition of 

 this work was published, Messrs. Powell and Lealand have succeeded 

 in making for me a one-fiftieth of an inch objective which magnifies 

 very much more highly than the twenty-fifth. This wonderful glass 

 was completed October i5th, 1864. It defines quite as well as the 

 twenty-fifth, and there is no difficulty in obtaining plenty of light for 

 the illumination of the objects (Proceedings of the Royal Society, 

 January iQth, 1865). Some drawings by this objective are given in 

 my Report on the Cattle Plague, and in pi. XXXVIII, fig. 244, and 

 pi. XL, fig. 254, of this work. There is less difficulty in bringing 

 this glass to a focus than would be supposed, although of course 

 delicate manipulation and some degree of patience and care are 

 requisite in working with it. 



359. The Apparent Size of an Object under different Powers. 

 The following circles give an idea of the size which objects would 

 appear in the microscope under the respective magnifying powers as 

 stated : 



Molecule ^^rnr of nn inc!l m diameter 



x 250 linear. . . 



Molecule 



o 



4U ' UO of an inch in diameter 

 x 700. 



The same x 7'- 



O 



The same x iSoo. 



The same x iSoo. 



O 



The same x 4000. 



o 



Molecule troW of an inch in diameter 



x 250 linear. The same x 3000. 



