316 SUMMARY OF CURRENT RESEARCHES RELATING TO 



on account of the rays from the Microscope having to undergo a 

 double total reflection, whilst the necessity of drawing with the head 

 in the same position as when using a vertical Microscope sacrifices 

 one of the great advantages of an inclined Microscope. This would be 

 remedied by reversing the prisms, so that the unreflected rays are 

 those which come from the Microscope and not from the paper, the 

 reflected rays being received from the paper. 



Relative merits of Filar and Ordinary Glass Eye-piece Micro- 

 meters.* — Dr. M. D. Ewell has undertaken a series of comparisons 

 to test Mr. H. L. Tolman's conclusion "j" that the cobweb micrometer 

 does not offer sufficient advantage in point of accuracy to compensate 

 for its additional cumbersomeness and expensiveness. 



Dr. Ewell comes to the conclusion that for the comparison of 

 lengths nearly equal and for the measurement of minute distances 

 with low powers, the glass eye-piece micrometer is vastly inferior to 

 the filar micrometer ; and that in cases where the greatest attainable 

 accuracy is required, as for example in the measurement of blood- 

 corpuscles in criminal cases, nothing but the filar micrometer should 

 be used. 



The New Objectives. — For some months past it has been known 

 that we were on the eve of an important advance in objectives, 

 depending mainly on the elimination of the secondary spectrum,, 

 leaving only a small tertiary spectrum. We alluded to the subject at 

 the Anniversary Meeting, by way of supplement to the remarks of the 

 President on the great value which he had found an increase of 

 aperture to be in his researches on very minute organisms with 

 high powers, and we expressed the belief that the new objectives 

 would be found to be of at least equal advantage. 



Two objectives have now been received in this country, and 

 their examination has fully borne out the expectation formed of 

 them, and has shown that however trifling the improvement might at 

 first sight be thought to be on theoretical grounds,^ it is very distinctly 

 appreciable, so that the high power work of the future will almost 

 necessarily be done with these glasses. 



The objectives in question are both 1/8 in. The special point in 

 their construction is that they are made of new kinds of optical 

 glass, which Prof. Abbe and Dr. Schott have been working for the 

 last five years to perfect. The objectives are composed of ten 

 single lenses, combined to five separate lenses, with a single front 

 lens. Their working distance is • 25 mm., and in order to secure 

 this the aperture is limited to 1-40 N. A. With the length of tube 



* The Microscope, vi. (1886) pp. 32-40. + See this Journal, v. (1885) p. 704. 



X Prof. Abbe writes us on this point, " We have now made what I called in 

 1878 ' the Microscope of the future,' i. e. objectives which admit of a more perfect 

 concentration of all the rnys from the object. If now (as I am nearly sure they 

 will) microacopists should feel somewhat disappointed at being told that 'the 

 Microscope of the future ' is nothing more and nothing better in principle than 

 these objectives, I must answer ' It is not my fault that at this time microscopical 

 optics is such an ungrateful domain of human work, that many years' hard labour 

 have no other result than a slight advance.' " 



