COKSTRUCTION OP THE MICBOSCOPE. 



33 



jected through a piece of flat parallel glass, with the various indices of 

 refraction for the different colours, it will be seen that each ray will 

 emerge, separated, into a beam consisting of the component colours of 

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

 ence, 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 im- 

 merse them in a fluid, they should be covered with the thinnest pos- 

 sible 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 correction for colour. 



" The mechanism for applying these principles to the correction of 

 an object-glass under the various circumstances is represented in fig. 27, 

 where the anterior lens is set in the end of a 

 tube a, which slides on the cylinder 6, contain- 

 ing the remainder of the combination ; the 

 tube a, holding the lens nearest the object, 

 may then be moved upon the cylinder b, for 

 the purpose of varying the distance, accord- 

 ing to the thickness of the glass covering the 

 object, by turning the screwed ring c, or more 

 simply, by sliding the one on the other, and 

 clamping them together when adjusted. An 

 aperture is made in the tube a, within which 

 is seen a mark engraved on the cylinder ; 

 and on the edge of which are two marks, a 

 longer and a shorter, engraved upon the tube. 

 When the mark on the cylinder coincides with the longer mark on the 

 tube, the adjustment is perfect for an uncovered object ; and when the 

 coincidence is with the short mark, the j)roper distance is obtained to 

 balance the aberrations produced by glass the hundreth of an inch 

 thick, and such glass can be readily supplied. This adjustment should 

 be tested experimentally by moving the milled edge, so as to separate 

 or close together the combinations, and then bringing the object to 

 distinct vision by the screw adjustment of the microscope. In this 

 process the milled edge of the object-glass will be employed to adjust 

 for character of definition, and the fine screw movement of the micro- 

 scope for correct focus. 



" It is hardly necessary to observe, that the necessity for this correc- 



D 



fig. 27 



