Dr. G. J. Stoney on Microscopic Vision. 429 



in the image of any sharp edges that may be upon the object, 

 owing to the exclusion of the beams B6. The exclusion of 

 these same beams also often produces a false glare, or adds 

 features to the image that are foreign to the object (Proposi- 

 tion 3, p. 345). We shall have to study of what kinds these 

 illusory effects are, and how they may be in some degree 

 controlled by adjustments of the illuminating apparatus. 

 There is another consequence of the exclusion of the beams 

 Ba and B6. It is apt to produce a deceptive colouration of 

 the images seen in the field of view. (Proposition 4, p. 345.) 

 The most important part of the study of microscopic vision 

 is the study of these matters. But before endeavouring to 

 go more fully into them, which can be best done in Part III., 

 we must take a glance at the subsequent and less important 

 steps that intervene between Standard Image No. 2 and the 

 Visual Image, which last is what is directly presented to the 

 eye of the observer. 



25. Transition from C to D. — It has been proved in the 

 footnote on p. 339 of Part I., that the illuminating apparatus 

 and the microscopic object may be removed, and that Standard 

 Image No. 1 can take their place. Now, for our present 

 purpose it will be enough to put Standard Image No. 2 in 

 their place ; since we are only concerned with the light which 

 is to be made use of by the objective, and this light remains 

 the same whether it is supplied by standard image No. 1 or 

 standard image No. 2. In order to be available for this use 

 of it, standard image No. 2 must be regarded as formed by 

 subjecting that portion of the light emitted by the object 

 which can be taken in by the objective to two successive 

 reversals ; and we shall find it convenient to conceive the 

 medium in front of the objective, whether oil or air, to be 

 continued downwards in order that the image may lie within 

 this medium, so that the light may be able to go straight 

 from the image to the objective. After the second reversal 

 the light first forms the image and then proceeds on from it 

 and enters the objective in precisely the same state as the 

 light actually emitted by the microscopic object would have 

 done. We may therefore remove the source of light and the 

 microscopic object, and substitute for them standard image 

 No. 2, transmitting the beams that form it on to the objective. 

 Each of these beams is a beam of parallel light, and is there- 

 fore, by passing through the objective, brought to a focus at 

 #, a situation which is usually close to the back lens of the 

 objective. See the figure on p. 433. After passing this 

 focus the beam diverges in the form of a cone of convex 

 spherical waves, and it is in this state that it reaches D, the 



