Royal Microscopical Society. 5 



The micrometer eye-piece for displaying at once the magnifying 

 power at a glance is thus constructed : focal length of field-glass, 

 3 inches ; focal length of eye-glass, fj inch ; distance between the 

 lenses, 2 inches; number of divisions of lens-micrometer, 100 per 

 inch. 



Observation. — The'aerial image of the spider-lines of Browning's 

 micrometer placed under the stage was then shown in the focus of 

 the microscope at about 200 diameters. Six full turns of the 

 divided milled head of the Browning micrometer moved a spider- 

 line image exactly 100th of an inch upon the stage micrometer, and 

 as the milled head was divided into 100 parts, one of its divisions 

 exactly corresponded to a movement of the aerial image -^ -f- 

 ^^o-th, or -g^o^th of an inch, and half a division to the -rWowth. 



Referring to the new " lens-micrometer " placed at the stop of 

 the Huyghenian eye-piece, all objectives can at once be examined 

 and their focal lengths assigned by simply remembering that all the 

 makers construct their objectives on the following principle ; if we 

 divide 100 by the assigned focal length, we have the magnifying 

 power of the microscope with " C eye-piece." 



Now in general the B eye-piece is half the power of the " C " and 

 D double. Moreover the £ of Andrew Boss is really a £, for it 

 magnifies 500 with C eye-piece instead of 400. In each case the 

 stop of the eye-piece is supposed to be exactly 10 inches from the 

 object on the stage. 



The " Lens-Micrometer " is an improvement upon the Kratometer 

 already described, p. 79, XXVI. of this Journal for 1870. 



(To be continued.) 



