358 



OBJECTIVES, EYE-PIECES, THE APERTOMETEE 



of a screw 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 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 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 



FIG. 312. Section of adjusting object-glass. 



FIG. 818. 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 * under-correction ' in the objective, fitted to 

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

 glass cover of extremest thickness. 



This method of collar correction served the purposes of micro- 

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

 tions were undertaken and objectives had more aperture given to 

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



The first was that the * covered ' and * uncovered ' marks were 

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

 fitfy divisions, a device introduced by James Smith in 1841 so that 



