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



17. Proposition 5. The Condenser. — The standard image 

 admits of being either better or worse. It manifestly admits 

 of being improved by forming it out of light of shorter wave- 

 length ; and this may be accomplished either by exchanging 

 the colour of the illumination employed for a colour of higher 

 refrangibility, or by mounting the object in a highly refracting 

 medium. 



But the degree in which the standard image correctly 

 represents the object usually depends even more upon the 

 condenser. In fact, the disturbance of the aether in front of 

 the object is determined partly by the features of the object 

 and partly by the condition of the light which illuminates it. 

 This is evident because if the reversal spoken of in §8 were 

 to take place without removing the object, the light in re- 

 tracing its steps would first reproduce the disturbed state that 

 had existed in front of the object ; would next form the 

 standard image upon the surface of the object ; and would 

 then pass through the object and form beyond it whatever 

 disturbed state of the aether had existed between the con- 

 denser and the object. Hence, that the standard image may 

 represent the features of the object unmixed with other 

 appearances not belonging to the object, it is essential that the 

 light provided by the condenser shall be as nearly uniform 

 and featureless as possible where it reaches the part of the 

 object which is being scrutinised. Hence the importance of 

 thin sections, and of a very well-corrected condenser. 



The management of stops, and their function, can be better 

 treated of in Part II. of this memoir, when we can enter into 

 details. For the present we content ourselves with a very 

 general proposition, viz. :— 



Peoposition 5. 

 The standard image is the outcome, partly of the features 

 upon the object, and partly of the state of the light by which the 

 object is illuminated. It may be improved by increasing the 

 degree in which the first of these factors, and by decreasing the 

 degree in which the second, contributes to produce, to modify, 

 or to efface detail in the image. 



18. Proposition 6. — When an object is mounted in a more 

 refractive medium than that in front of the objective, standard 

 image No. 1, which depends on the wave-length of the light 

 as it quits the object, is thereby improved ; but standard image 

 No. 2 is not enabled to grasp any finer detail upon the object 

 than it would have grasped if the object had been in a medium 

 of the same refractive index as that in front of the objective. 



