353 



OBJECTIVES, EYE-PIECES, THE APEKTOMETEK 



of ii sereu arrangement, as in fig. 312. The front pair of lenses is 

 fixed into a tube (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 ciillar (I 1 ), 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 oblong 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 horizontal lines are 

 engraved, one pointing to the word 'uncovered,' the other to the 

 word ' covered ; ' whilst the latter is crossed by a horizontal mark, 

 which is brought to coincide with either of the two lines by the 

 rotation of the screw-collar, whereby the outer tube is moved up 

 or down. When the mark has been 

 made to point to the line ' uncovered/ 

 it. indicates that the distance of the lenses 



Uncovered. 

 Covered. 



FIG. 312. Section of adjusting object-glass. 



FIG. 813. Present collar 

 correction. 



of the object-glass is such as to make it suitable 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 such proximity with the other two 

 as to produce an Minder-correction' in the objective, fitted to 

 neutralise the 'over-correction' produced by the interposition of a 



glass cover of ext remest thickness. 



This method of collar correction served the purposes of micro- 

 scopy for upwards of 1 hilly years, but when more critical investiga- 

 tions were undertaken and objectives had more aperture giA'eii to 

 them it uas found that the method had two great faults. 



The lir.st uas that the 'covered' and 'uncovered' marks were 



11 crude. To remedy this, the screw collar was graduated into 



litly dixisions. a device introduced by .lames Smith in 1841 so that 



