7 i 2 SUMMARY OF CURRENT RESEARCHES RELATING TO 



(4) Photomicrography. 



Elementary Photomicrography.* — W. Bagshaw's book, with the 

 above title, has now reached its second edition. It is a work expressly 

 intended for the beginner, and endeavours to encourage him instead of 

 to dismay him. With this purpose in view, the author keeps strictly 

 within suitable limits, and the beginner will, therefore, rind exactly the 

 help he requires. The instructions are all very clearly and unmistakably 

 set forth, and no one attempting to follow them is likely to fail. The 

 illustrations include examples of several beautiful objects. 



(5) Microscopical Optics and Manipulation. 



Microscopical Image Formation.! — Under the above title, F. J. 

 Keeley discusses the present views on the theory of microscopic vision. 

 He points out that there are two schools, the " diffraction " and the 

 " dioptric." Although both of these have to deal with diffraction, the 

 latter concerns itself only with such as arises within the instrument 

 itself : the former attaches pre-eminent importance to diffraction 

 originating in the object under examination. The author's aim is to 

 study both theories in an unprejudiced manner supplemented by experi- 

 ments, with the view of endeavouring to learn whether they are wholly 

 irreconcilable. Up to the time of Abbe, the dioptric view had been 

 unquestioned. This view, however, has had to give way to Abbe's 

 theory, which has stood unrefuted up to the present time. But, recently, 

 a feeling has been aroused that, without further modification, it fails to 

 fully account for all features of observed microscopical images. Wright's 

 "Principles of Microscopy," published in 1906, fully restates and ex- 

 pounds the old dioptric view, and performs its task so well that Keeley 

 finds but one point open to criticism, viz., the employment of bright 

 points and lines, as equivalent to self-luminous objects in explaining 

 the formation of the diffraction patterns termed "anti-points." For 

 the purpose of definitely testing the effects of aperture, he made a 

 very careful series of observations with high-power objectives on two 

 well-stained human blood corpuscles. Full details of the experiments 

 are given by him, the N.A. varying from 1*20 to "20. The variations 

 in the results were well within the limits of experimental error, whence 

 the author concludes that the unquestionable presence of " anti-point " 

 phenomena need not be taken into consideration in connection with 

 micrometry with white light, as a trained eye can select the same 

 margins to measure under any ordinary conditions of illumination. 

 This is not in the least contradictory to the theory, but demonstrates 

 the possibility of overcoming a theoretical difficulty in actual practice. 



In considering diffraction phenomena originating in the object, it 

 will be well to first assume conditions under which the objective will be 

 of greater aperture than the illuminating cone, and will, therefore, when 

 examined with ocular removed, exhibit a disk of light, the dioptric 

 beam, surrounded by an unilluminated space. The insertion of an 

 object in the focus of the objective will result in this dark space showing 



* Ilift'e and Sons, London, 1909 (45 illustrations), 



t Proc. Acad. Nat. Sci. Philadelphia, lxi. (1909) pp. 177-92. 



