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



surface is embarassed by somewhat similar difficulties to those 

 we meet with when we try to look through ribbed glass. 

 Thus, on the siliceous diatom known as Pinnalaria nohilis, it 

 is easy to make out that there are rows of dots on the tongues 

 that stand inwards towards the median line; but vision is 

 here mixed up with so many false effects, that it would be 

 difficult to determine how many of these rows there are and 

 where exactly they are placed. All will probably be reduced 

 to order if the diatom can be mounted between two media, of 

 which the under one shall have a refractive index of 1*4, the 

 same as silex, and the upper one a refractive index as much 

 as possible exceeding 1*4. To mount objects in this way 

 ought not to be impracticable. 



22. The Six Images. — When a particular mode of illumina- 

 tion has been adopted, all subsequent events are thereby 

 determined if the optical parts of the microscope are fully 

 corrected and properly adjusted. To trace these events from 

 the object on the stage to the image of it delineated upon the 

 retina of the observer is now our task ; and it may be prose- 

 cuted by beginning with an ideally perfect image and applying 

 in succession a series of changes which finally transform it 

 into the image which actually presents itself within the eye 

 of the observer. Throughout the whole inquiry we may, 

 without loss of generality, treat only of the light of one wave- 

 length out of the light employed. 



The following is a list of the successive stages of the trans- 

 formation. In reading it the diagram on p. 433 will be found 

 of use : — 



A. Object A is the actual microscopic object, to which 

 corresponds Image A, viz. : that ideal image reproducing 

 all possible detail, which only light of infinitesimal wave- 

 length supplied in a theoretically perfect manner would 

 be competent to produce. 



B. Image B, the standard image, means that image 

 which the light of wave-length X as actually emitted by 

 the microscopic object would furnish, if reversed (see 

 Part I. section 8, p. 338). This image is of the same 

 size as the microscopic object. To it would correspond 

 Object B_, an object of the same size as Object A, but 

 differing from it by containing no detail but that shown 

 in Image B. 



C. Image C, standard, image No. 2, is that which 

 the light taken in by the objective would, if reversed, 

 produce; to which would correspond Object C, an object 

 containing the same detail as that shown in Image C. 



