34 THE MICROSCOPE. 



line of the Podura and other delicate scales; and if diverging 

 rays near the axis and at the margin are projected through a 

 piece of flat parallel glass, with the various indices of refraction 

 for the different colors, it will be seen that each ray will emerge 

 separated into a beam consisting of the component colors of 

 the ray, and that each beam is widely different in form. This 

 difference, being magnified by the power of the microscope, 

 readily accounts for the chromatic thickening of the outline 

 just mentioned. Therefore to obtain the finest definition of 

 extremely delicate and minute objects, they should be viewed 

 without a covering; if it be desirable to immerse them in a fluid, 

 they should be covered with the thinnest possible film of talc, 

 as, from the character of the chromatic aberration, it will be 

 seen that varying the distances of the combinations will not 

 sensibly affect the correction; though object-lenses may be 

 made to include a given fluid or solid medium in their cor- 

 rection for color. 



"The mechanism for applying these principles to the cor- 

 rection of an object-glass under the various circumstances, is 

 represented in Fig. 18, where the anterior lens is set in the end 



Fig. 18. 



