466 



MANIPULATION. 



focal length, which transmitted angular pencils of 60" and 72o. 

 About this time, in conjunction with Mr. Lister, he con- 

 structed an inch object-glass of two combinations, the form of 

 the front lens being suggested by Mr. Lister himself; this 

 glass was capable of transmitting an angular pencil of 22". 

 At this time he also constructed one-eighths, the front glasses 

 of which were also of the form suggested by Mr. Lister; 

 these had an aperture of 63" and 64"; he continued making 

 these last untU the year 1842, when he increased the angle of 

 aperture of the half-inch to 44", of the quarter to 63", and the 

 one-eighth to 74". In the year 1844 Professor Amici visited 

 this country, and brought with him an object-glass of one- 

 seventh of an inch focal length, with an aperture of 112"; this 

 combination was in part composed of Dr. Faraday's dense glass. 

 Mr. Ross copied Amici's construction; but found the dense 

 glass so exceedingly soft and fragile, as to render it unfit to 

 receive the high polish so essential to the correct performance 

 of any object-glass : he also noticed that Amici's glasses were 

 much tarnished ; he then devised a new construction, whereby, 

 with the ordinary dense glass, he obtained an aperture for 

 pencils in the one-eighth of 85", and in one-twelfth as high as 

 135", the largest angular pencil that can be passed through a 

 microscopic object-glass." As now constructed, the angular 

 apertures, with the greatest separating magnifying power of 

 object-glasses of different focal lengths, are represented in the 

 following table: — 



The test objects now generally employed for ascertaining 



