CONSTRUCTION OF OBJECTIVES. 47 



observed through a film of air ; " and again, " Consequently 

 we have a loss of aperture when an air angle of 180 is 

 substituted for a balsam-angle of ioo," for " an immersion 

 objective of balsam angle exceeding twice the critical angle 

 (41) has a greater aperture than any dry lens can ever 

 have." It was upon this subject that Mr. Wenham fell into 

 error and so may be classed with Ross, Goring, Wollaston, 

 and Biot. He denied that Tolles had produced, or could 

 ever produce an objective of greater balsam angle than was 

 equivalent to infinitely near 180 measured in air ; and those 

 who remember the correspondence on this subject which 

 appeared in the pages of the ' Monthly Microscopical 

 Journal/ will now smile when they see the announcements 

 in the 'Journal of the Royal Microscopical Society ' of the 

 productions of Messrs. Powell and Lealand of 1 50 balsam 

 angle. That memorable correspondence should teach us 

 to remember how easy it is to fall into error, and also that 

 it is quite easy to persuade ourselves we are well acquainted 

 with our subject, while at the same time we may totally 

 misunderstand it. 



Objectives which, by-the-by, are sometimes called 

 " powers " being made from glass of varying density and 

 also of varying refractive indices, it follows that they must 

 differ also in construction in some degree. The various 

 lenses of which each combination is constructed are ground 

 to a series of curves, suitable to the glass employed, and 

 the combinations are placed at different distances apart, so 

 that we can only give a rough outline of their general 

 construction. 



As a rule, objective mounts are turned out much too 

 long. There is no apparent reason why the brasswork 

 (of some opticians especially) should not be consider- 

 ably reduced. When the posterior lens is too far away 

 from the Wenham prism, in a binocular instrument, it is 



