Ohservations on Amphiiyleura pellucida. By J. J. Woodiuard. 667 



glass (it has been the fashion to call it the balsam angle), could 

 by no means exceed double the angle of total reflexion from glass 

 to air, which theory indicated as the hmit for dry objectives. It 

 is now universally admitted by the physicists and mathematicians 

 whose attention has been directed to the subject, that this limit, 

 inflexible as it necessarily is for dry objectives, has nothing what- 

 ever to do with the question of the aperture of immersion objectives, 

 which indeed, from this point of view, would be limited only by 

 double the angle of total reflexion from glass to the immersion 

 fluid. The adherence of any individual microscopist to the old 

 error is therefore no longer of importance, and the practical opti- 

 cians, acting in strict accordance with correct theory, have succeeded 

 in constructing water, glycerine, and oil-immersion objectives, not 

 merely exceeding 82°, but attaining 100^, 115°, and even, in the 

 case of one well-known maker, over 120^ balsam angle for an oil- 

 immersion objective. The limit of improvement in this direction 

 is not yet attained. 



Professor Abbe has recently announced his expectation * that it 

 wiU be possible in the near future to construct objectives having an 

 aperture of 1'28° in a medium of 1 "50 index of refraction. That 

 this expectation will be realized, and with resulting improvement 

 in defining power, I do not doubt in the least, but cannot believe 

 that the limit will be found even here. 



But while the correct views with regard to the aperture question 

 are now generally accepted, and a number of practical opticians 

 are making use of this knowledge in the construction of improved 

 objectives, we hear it continually asserted of late, that no advantage 

 can possibly be derived from the excess of angle in immersion objec- 

 tives beyond the much-talked-of 82°, unless immersion illuminators 

 are used to throw the illuminating pencil upon the object with an 

 increased obhquity corresponding to the increased angle of the 

 objective. 



This statement, which is an exceedingly exaggerated one, has 

 been so loudly reiterated, and has been accepted, without special 

 investigation, in such high quarters, that I have thought it worth 

 while to take this opportunity of drawing attention to the actual 

 facts. It is quite true that when a feeble source of illumination, 

 such as a coal-oil lamp, is employed, it is necessary to use an immer- 

 sion illuminator by which the light can be thrown upon the object 

 more obhquely than would otherwise be possible, whenever we 

 desire to obtain the best possible resolution of difficult lined test 

 objects with objectives whose balsam angle exceeds 82°. It is also 

 true that the performance of these objectives on histological and 



* ' Ueber Stephenson's System der homogenen Immersion bei Mikroskop- 

 Objectiven,' " Sitzungsberichte der Jenaischen Gesellschaft fiir Medicin uad 

 Naturwissenschaft," Jan. 10, 1879. 



