MAGNIFYING POWERS. 175 



separated into a beam consisting of the component colours 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 fluid, they should be covered with the thinnest possible 

 film of mica or glass, 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 correction for colour." 



The mechanism for applying these principles to the correc- 

 tion of an object-glass under the various circumstances, has 

 been already described at page 42, where also was shown, by 

 fig. 22, a vertical section of an achromatic object-glass, of one- 

 eighth of an inch focus ; the mode of making the adjustments 

 according to the plan of Mr. Eoss, as well as that of Mr. 

 Wenham, which has only lately been proposed, will be fully 

 described in the chapter devoted to test objects. 



" It is hardly necessary to observe," says Mr. Ross, " that 

 the necessity for this correction is wholly independent of any 

 particular construction of the object-glass ; as in all cases 

 where the object^lass is corrected for an object uncovered, 

 any covering of glass will create a different value of aberration 

 to the first lens which previously balanced the aberration 

 resulting from the rest of the lenses ; and as this disturbance 

 is effected at the first refraction, it is independent of the other 

 part of the combination. The visibility of the effect depends 

 on the distance of the object from the object-glass, the angle 

 of the pencil transmitted, the focal length of the combination, 

 the thickness of the glass covering the object, and the general 

 perfection of the corrections for chromatism and the oblique 

 pencils." 



The object-glasses that are supplied with the adjustment, 

 for thickness of glass, are the highest powers, viz., the one- 

 sixteenth of an inch in focal length, the one-eighth, the one- 



