ADJUSTMENT OF THE OBJECT-GLASS. 



165 



which the more remote (it being the special result of the ordinary 

 mode of viewing objects by transmitted light, that such differ- 

 ences are obliterated), unless, by the use of the " fine movement," 

 it be ascertained, when they are successively brought into focus, 

 whether the object-glass has been moved towards or away from 

 the object. Even this, however, will not always succeed in cer- 

 tain of the most difficult cases, in which the difference of level 

 is so slight as to be almost inappreciable ; as, for instance, in 

 the case of the markings on the siliceous loricw of the Diatomacese 

 (Fig. 80). 



83. When objectives of short focus and of wide angular aper- 

 ture are being employed, something more is necessary than exact 

 focal adjustment; this being the Adjustment of the Object-glass 

 itself, which is required to neutralize the disturbing effect of the 

 glass cover upon the course of the rays proceeding from the ob- 

 ject ( 15). For this adjustment, it will be recollected, a power 

 of altering the distafcce between the front pair and the remainder 

 of the combination is required; and this power is obtained in 

 the following manner. The front pair of lenses is fixed into a 

 tube (Fig. 53, A), which slides over an interior tube (B) by which 

 the other two pairs are 

 held ; and it is drawn up 

 or down by means of a 

 collar (c), which works in 

 a furrow cut in the inner 

 tube, and upon a screw- 

 thread cut in the outer, 

 so that its revolution in 

 the plane to which it is 

 fixed by the one tube, 

 gives a vertical movement 

 to the other. In one part 

 of the outer tube, an ob- 

 long slit is made, as seen 

 at D, into which projects 

 a small tongue, screwed 

 on the inner tube ; at the 

 side of the former two 



Section of an Adjusting Object-Glass. 



horizontal lines are engraved, one pointing 



the word " covered ;" whilst the latter is 



, . , to the word "un- 



covered," the other to 

 crossed by a horizontal mark, which is brought to coincide with 

 either of the two lines by the rotation of the screw-collar, which 

 moves the outer tube up or down. When the mark has been 

 made to point to the line "uncovered," it indicates that the dis- 

 tance of the lenses of the object-glass is such, as to make it suita- 

 ble for viewing an object without any interference from thin 

 glass ; when, on the other hand, the mark has been brought, by 

 the revolution of the screw-collar, into coincidence with the line 

 " covered," it indicates that the front lens has been brought into 



