244 SUMMARY OF CURRENT RESEARCHES RELATING TO 



Tn the second chapter the image-formation of self-k;minous objects 

 on the principle of the wave theory is considered. This is followed by 

 the literature of those cases in which strict calculation on Maxwell's 

 light theory has succeeded with diffraction phenomena. 



The third chapter is devoted to tlie image-formation of non-self- 

 luminous objects. 



The subject of the fourth chapter is the image-formation of a grating 

 with artificial outline, and embraces the main results of a doctoral disser- 

 tation undertaken by M. Wolfke at the author's request, and dealing 

 with a grating illuminated by a plane wave perpendicularly incident 

 upon it. The dissertation does not extend to the case of a self-luminous 

 grating. 



The book concludes with a bibliography on the theory of imao-e- 

 formation of non-self-luminous objects. 



Microscopical Work and Eye-accommodation.* — F. Brocher has 

 investigated the amount of accommodation which an observer's crystalline 

 lens exerts during microscopical work. He uses a camera lucida at- 

 tached to the Microscope in the usual way, but the ordinary sheet of 

 drawing-paper is replaced by a mirror, set at such an angle as to reflect 

 the landscape. On this mirror, in the path of the Microscope rays, he 

 places a small piece of postage-stamp paper. A person looking through 

 the camera lucida now sees three images, viz. the object (an engraved 

 scale) on the Microscope stage, the landscape, and the fragment of paper. 

 Two of these images (the scale and the landscape) would by many people, 

 especially by experienced microscopists, be clearly seen, whilst the paper 

 image would be indistinct. As the landscape is a distant object, normally 

 visualized with the crystalline lens completely relaxed, the conclusion 

 follows that the Microscope image is seen under similar conditions, or, in 

 other Avords, the eye functions microscopically without accommodation. 

 If the observer's eye is normally conformed, an accommodation of about 

 2 • 5 dioptrics would bring the stamp-paper into view, but the other two 

 images would become indistinct. These observations seem to explain 

 why it is that many persons can work with the Microscope for prolonged 

 periods without eye-fatigue. 



The author found, however, a second class of observers who do com- 

 plain of eye fatigue, and these were usually comparatively inexperienced 

 microscopists. In their case the same apparatus was used, but the two 

 images clearly seen were now the scale and the stamp-paper : the land- 

 scape was indistinct. Thus it follows that eye-accommodation was exerted. 

 By measurements the author estimated the magnitude of this accommo- 

 dation, and found that it was not necessarily the maximum of which the 

 eye is capable. 



It is evident that a person of the first class wishing to use the camera 

 lucida for drawing could not focus the Microscopic image on the paper 

 without eye-accommodation. Such an one should place above the prism 

 a lens whose refractive power is equal to the accommodation-effort neces- 

 sary to see the paper. 



Observers of the second class with practice frequently acquire in- 



* Kev. M6d. Suisse Eomande, xxxi. (1911) pp. 69 and 84 (2 figs.). 



