428 Dr. G. J. Stoney on Microscopic Vision, 



by the convergence of two or more of the above-mentioned 

 beams of plane waves, or of components of them. The finest 

 of these rulings are those formed by beams that flow in 

 horizontally from opposite longitudes. If each of the beams 

 is resolved into light polarized in, and light polarized perpen- 

 dicularly to the plane of incidence, then the opposed pair of 

 each of these components will produce a ruling in which the 

 brightness varies by the law represented by the expression 

 1 H- cos (47T#/\). This indicates that it is a ruling of which the 

 spacing is \/2, each space including the width of a line and 

 of the interval between it and the next. Such a ruling will 

 be best seen when the intensity of the illumination is such 

 that the lines and intervals appear of equal width. Hence the 

 lines of the finest rulings, and consequently any speck in the 

 image seen by the interlacing of these rulings, will have an 

 apparent diameter of X/4:, when best seen. 



Now the corresponding portion of the microscopic object, 

 viz. a globe with \/4 for its diameter, contains wdthin it an 

 enormous quantity of detail, all of which is massed together 

 in the standard image and presented to us as one mere speck. 

 How great a loss is here incurred, and how little any micro- 

 scope can exhibit to us of nature, may be judged from a 

 numerical example which will be given in Part 111.* 



24. Transition from B to C. — The important change from 

 B to C requires to be carefully studied from many sides. 

 Standard image No. 1 contains everything which the whole 

 of the light of wave-length \ is, in the condition in which it 

 leaves the object, capable of presenting. Standard image 

 No. 2 contains the utmost that can be delineated by that por- 

 tion of this light which the objective is able to take in. 



The change from the first of these to the second involves, 

 in the first place, some obliteration of detail, consequent upon 

 the exclusion of the inclined beams Ba, which are spoken of 

 on p. 344, in § 15 of Part I. It also involves a rounding off 



* The outcome of the computation here referred to is that with bluish- 

 green light, which has probably the shortest wave-length that can be 

 used with advantage in eye observations, and when its wave-length is 

 shortened by mounting the object in a medium with as high a refractive 

 index as immersion oil : under these favourable circumstances, a spherical 

 portion of the object of the size of the smallest speck which this light is 

 capable of exhibiting, would still be large enough to contain something 

 between 300,000 and 300,000,000 of those chemical atoms of which pon- 

 derable matter is made up. And we should remember that the immer- 

 sion objective for which the oil is designed cannot exhibit quite so small 

 a speck as the smallest which light of that wave-length could produce. It 

 is a limit beyond its grasp. Biologists would do well to ponder what 

 computations of this kind teach us. 



