THE MIOBOSCOPE. 35 



of a tube A A, which slides on the cylinder B containing the re- 

 mainder of the combination; the tube A A, holding the lens 

 nearest the object, may then be moved upon the cylinder B, 

 for the purpose of varying the distance according to the thick- 

 ness of the glass covering the object, by turning the screwed 

 ring C 0, 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 proper distance is ob- 

 tained to balance the aberrations produced by glass one- 

 hundredth of an inch thick, and such glass can be readily sup- 

 plied. 



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

 correction is wholly independent of any particular construction 

 of the object-glass; as in all cases where the object-glass is cor- 

 rected for an object uncovered, any covering of glass will create 

 a different value of aberration to the first lens, which previ- 

 ously 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 correc- 

 tions for chromatism and the oblique pencils. 



"With this adjusting object-glass, therefore, we can have 

 the requisites of the greatest possible distance between the ob- 

 ject and object-glass, an intense and sharply defined image 

 throughout the field from the large pencil transmitted, and the 

 accurate correction of the aberrations; also, by the adjustment, 

 the means of preserving that correction under all the varied 

 circumstances in which it may be necessary to place an object 

 for the purpose of observation." 



In the annexed engraving, Fig. 19, we have shown the triple 

 achromatic object-glass in connection with the eye-piece con- 



