ON OPTICAL aUALTTY OF MICROSCOPE OBJECTIVES. ^ 



working capacity of his instrument, measuring this by a standard 

 of strict optical requirements. The advantage of substituting 

 some such proceedings for the comparative trial of lens against 

 lens so long in vogue can scarely be disputed. For although the 

 best warrant of a well constructed lens is the fair reputation of its 

 maker J and the choice of an objective resolves itself for the most 

 part into the selection of the particular make of one or other of 

 the best accredited opticians, still when the instrument is 

 purchased its possessor frequently becomes haunted by the desire 

 to pit its performance against that of some neighbour's 

 instrument, or to match the performances traditionally accepted in 

 our handbooks. A short and easy method of testing an objective, 

 not by comparison with others only, but by itself and on its own 

 merits, affords not only the most direct and positive evidence of 

 its qualities to those who are more concerned in proving their 

 instruments than using them, but also yields to the genuine 

 worker the satisfying conviction that his labour is not frustrated 

 by faulty construction and performance of his instrument. It is, 

 however, to be borne in mind that the microscopist in any 

 scrutiny of the quality of his lenses which he may attempt, has no 

 other object in view than to acquire such insight into the optical 

 conditions of a good performance as will enable him to make the 

 best use of his instrument, and acquire confidence in his inter- 

 pretation of what he sees, as well as manipulative skill in 

 examining microscope objects. To the constructor and expert 

 in optical science, are left the severer investigations of optical 

 effects and causes, the difficulties of technical construction, the 

 invention of new lens combinations, and the numerous methods 

 of testing their labours by delicate and exhaustive processes which 

 require special aptitude, and lie entirely outside the sphere of the 

 microscopist's usual work. 



The mode of testing the optical power of an objective to which 

 we now invite attention, is that devised by Professor Abbe, and 

 described in an essay already translated and printed in our 

 Proceedings. In the present article further explanation is given. 



