Dr. Gr. J. Stoney on Microscopic Vision. 343 



vision, whether microscopic, telescopic, or with the naked 

 eye, is in fact the study of what this visual substitute is and 

 how it stands related to the real object, i. e., what alteration 

 the real object would have to undergo to be transformed into 

 its visual substitute, which is what seems to us to be the 

 object presented to us. 



The real object, 0, sends forward the light which enters the 

 eye, and, in addition, other light which does not enter the eye, 

 whereas its visual substitute, S, is to be defined as that other 

 object from which would emanate the light which enters the 

 eye and it only. It is evident that objects and S will seem 

 to us exactly alike, but that whereas we receive the whole of 

 the light which S is competent to dispense, we receive only a 

 part of that emitted by 0. Similarly, when we use a micro- 

 scope or telescope, what we seem to see is a visual object, C, 

 which would emit exactly the light which the eye takes in, 

 and it only ; and this is in all cases less than the light which 

 an enlargement of the object would emit, and may differ from 

 it in other respects also. It is, accordingly, to the study of 

 what these visual substitutes are that we s*hould apply our- 

 selves. 



But as this is a branch of optics which is as yet almost 

 wholly unexplored *, we must, for the present, be content 

 with the inferences we can draw from such general considera- 

 tions as the following : — 



15. Proposition 3. — The objective of a microscope has an 

 angular aperture which is necessarily less than 180°. Hence 

 the image formed by it is formed by a part only of the light 

 emitted by the object. 



Imagine a hemisphere in front of the object, of so large a 

 size that the whole object may be treated as though it were 



* In one simple case investigated by the writer the visual substitute 

 for a thin line of light proved to be a double line with a narrow interval 

 and ■with very thin appendage-lines on either side. Here we have some 

 of the phenomena presented by microscopes— a spurious resolution into a 

 double line, and appendage-lines which are of the same nature as inter- 

 costal markings. See abstract of communication to British Association, 

 at p. 583 of the < Report ' for 1894. 



