Dr. G. J. Stoney on Microscopic Vision, 425 



those features upon the object which we wish to examine, and 

 at the same time such as will exclude or minimise the addi- 

 tion to the microscopic image of false or confusing effects. 

 How this may best be aimed at will have to be considered 

 in Part III., and will be illustrated there. 



The source of light and the condenser are to be esteemed 

 as good in the degree in which they enable the whole of the 

 light of each wave-length which is brought to bear upon a 

 point of the object to reach that point at each instant in the 

 same phase and with accordant transversals. For if this con- 

 dition could be fully attained the state in which the light would 

 then leave the object would be complicated by the previous con- 

 dition of the light only in respects which cancel one another 

 when averaged over a sufficient period of time. For this average, 

 300 metres of cosmic time * — the millionth of a second — 

 would be abundant. There is, therefore, no " twinkling of an 

 eye " within which the average has not been struck. Hence 

 light so supplied will furnish a pure image so far as man can 

 see — one depending solely on the features of the object. 



The efforts to reach this desirable result seem to have been 

 directed exclusively towards improving the condenser, and 

 cutting very thin sections ; but both theory and experiment 

 seem to show that when the condenser is good, a further 

 advance may be made by attending to the source of light, 

 which apparently ought to be confined to a layer of a thick- 

 ness small compared with a wave-length, and preferably lying- 

 in a plane, or rather on a slightly concave surface, perpen- 

 dicular to the optic axis (see § 27 below, p. 435). Of course 

 the distance of the source of light is not immaterial, since a 

 well-corrected condenser acts at its best only when the source 

 of light is at one particular distance. 



However well the light may have been prepared by the 

 condenser, it is sometimes thrown into confusion before it 

 reaches the upper surface of the object, by having to enter 

 through its under surface and to traverse its substance. In 

 such cases the attempt to ascertain what is on the upper 



* Cosmic time means time the portions of which are measured by 

 lengths — by the distances over which electromagnetic waves in the open 

 sether, and therefore over which light in vacuo, would travel in those 

 portions of time. This way of measuring time is convenient in both 

 optics and molecular physics. The convenience has its foundation in 

 nature, as it no doubt arises out of the circumstance that this way of 

 measuring time brings both time and space into the closest possible asso- 

 ciation in which they can stand to the fundamental units of nature. See 

 a paper u On the Physical Units of Nature " in the Proceedings of the 

 Royal Dublin Society of Feb. 16, 1881, or in the Philosophical 'Magazine 

 of May 1881. 



