INVEETEBEATA, CEYPTOQAMIA, MICEOSCOPY, ETC. 515 



It is obvious, in fact, that the peripheral portion of the aperture 

 conducts very much fewer rays of light into the objective in pro- 

 portion to the central parts than would correspond with the measure 

 of their angles, and that consequently the angle of aperture cannot 

 be a correct expression of the real aperture — that is, of the capacity of 

 the optical system to receive light. This leads us empirically to the 

 conclusion which results from theory, viz. that " numerical aperture " 

 is the rational measure of aperture, and the only one from which an 

 estimate of its practical cfTccts can be formed. 



Systematic Examination of Objectives for the Microscope.— 

 Dr. G. E. Blackham, of Dunkirk (N.Y.), U.S.A., in a paper read at the 

 last (Buffalo) meeting of the American Society of Microscopists, says 

 that the microscopist dealing with minute quantities, is peculiarly 

 liable to be aided or hindered by slight differences in the quality of 

 his instruments ; for while it is doubtless true that an expert can do 

 very fair work witla instruments of only moderate quality, and the 

 tyro cannot do first-class work with instruments even of the most 

 superlative excellence, it is also true that, other things being equal, 

 the better the instrument the more reliable the results obtained, and 

 that the best work cannot be done even by the most expert worker 

 except with the best instruments. The comparison of instruments 

 must therefore always, if properly and skilfully done, be of value to 

 the worker, who may thus be enabled to select the most fitted for his 

 work. 



The most important optical j^art of the Microscope, and that most 

 subject to variation of quality, is the objective ; but the ordinary 

 methods of describing tliese seem to the author to lack precision. 

 For instance, we often see the recommendation to beginners to purchase 

 a stand and a 1-inch and a 1-inch objective. On referring to the 

 catalogue of some makers we may find the latter thus described : — 

 Student's series, i-inch, ap. 100°, non-adj., price S15 ; in another, 

 Student's series, 1, ap. 85°, non-adj., price ^12. How is the student 

 to choose between them ? 



The descriptions are insufficient to enable him to compare them. 

 Which glass has the greatest resolving power, which the flattest field, 

 which the longest working distance, and wliich the greatest amplifying 

 power? Of course if objectives were all truly named the last question 

 would bo answered, as all I's would have the same amplifying 

 power, viz. x 50 at 10 inches from optical centre of the objective, but 

 such is not the case. 



In order to meet this requirement and answer these questions, 

 Dr. Blackham has claboratc^d a system of examination and prepared a 

 form for recording the results, which gives at a glance most, if not all, 

 tlie points necessary to the coniparis<»u of objectives. 



The first coliunn gives tlie record number for convenience of 

 reference, the second the owner's name, and the third the maker's and 

 the year in whicli made. This latter is important, as in these days of 

 rai)id improvement in construction, it wouUl be obviously unfair to 

 compare an objective made five or even three years ago with ouo of samo 

 nominal grade made this year, without stating the fact. 



