37- JULIUS RHEIXBERG ON STEREOSCOPIC EFFECT AND A 



in one azimuth (the relative pros and cons of which we need not 

 stop here to consider) we frequently can gain some help in our 

 estimation of the form, and the same assistance may be afforded 

 by the use of suitable colour illumination.* 



We have now to consider causes 3 and 5, both of which play 

 an important rdh in vision with the microscope. Considering 

 the latter first — viz. the way in which the relative clearness or 

 haziness of different layers affect our estimation of their distance 



• in one another — I am not sure that this has been recognised 

 sufficiently ; but the simple experiment of taking, say, a watch 

 movement in which a number of thin wheels are visible one 

 behind the other, and regarding this with a 1-in objective stopped 



x\\ to various degrees by an iris diaphragm behind it, will 



quickly convince any one what a difference this makes. As soon_ 



the stopping down of the objectives lessens the size of the 



diffusion discs of points in the out-of -focus layers, these seem to 



come appreciably closer to the one in true focus. 



The accommodation of the eye is brought into play with the 

 microscope just the same as in ordinary vision. This subject, 



gether with the question of depth of focus of objectives, was 

 exhaustively investigated by Abbe, and his results were given 

 in a paper in the Zeltschrift far MiJcroskopie,^ which has been 

 admirably summarised at some length in Carpenter's Handbook. % 

 It will only be necessary here to touch upon some of the principal 

 points. 



It was shown that in vision with the microscope the whole 

 penetrating power was due to depth of focus of the instrument, 

 plus that due to the accommodating power of the eye. The- 

 focal depth of the objective depends upon the size of the diffusion 

 discs which may be considered permissible; if the visual 

 angle under which they are seen is about 3' of an arc, the 

 image is fairly distinct; if more than about 5', the image- 

 becomes too indistinct. The size of these discs, as we have 



" On an Addition to the Methods of Microscopical Research by a New 

 Wi ; y producing Colour-contrast between an Object and its 



md, or between Definite Parts of the Object itself," J. Rheinberg,. 

 i Royal Microscopical Society, 1896, p. 384. 



eines neuen stereoscopischen Oculars," Zeitschrift fur 

 Mikroskojtie, pp. 216-225. Vide also Journal Royal Microscopical Society.. 



X »o. 



