137 



screw-micrometer, being much handier to use, and not liable to 

 derangement in inexperienced hands. 



The next subject for consideration is the kind of eye-piece best 

 suited to microscopic measurement. 



The positive eye-piece was constructed by ftamsden especially for 

 micrometric use, and Mr. Ross applies it to the micrometers that he 

 furnishes to his microscopes. Mr. Smith and Mr. Powel, following 

 Mr. Tully, place the micrometer in the negative eye-piece in the 

 situation of the stop. 



Each of these plans has its peculiar advantages. The positive 

 eye-piece gives the best view of the micrometer, the negative of the 

 object. The former is quite free from distortion, even to the edges of 

 the field, but the object is slightly coloured. The latter is free from 

 colour, but is slightly distorted at the edges : in the centre of the 

 field, however, to the extent of half its diameter, there is no per- 

 ceptible distortion ; and the clearness of the definition gives a pre- 

 cision to the measurement which is very satisfactory. For this 

 reason I give it the preference. 



I will now proceed to describe the micrometer which I laid before 

 the Society at a former meeting. 



Short bold lines are ruled on a piece of glass ; and, to facilitate 

 counting, the fifth is drawn longer, and the tenth still longer, as in 

 the common rule. Very finely levigated plumbago is rubbed into the 

 lines to render them visible ; and they are covered with a piece 

 of thin glass, cemented by Canada-balsam, to secure the plumbago 

 from being wiped out. The slip of glass, thus prepared, is placed in 

 a thin brass frame so that it may slide freely ; and is acted on at one 

 end by a pushing-screw, and at the other by a slight spring (fig. 1). 



(Fig. 1). 



Slits are cut in the negative eye-piece on each side (fig. 2), so that 

 the brass frame may be pressed across the field in the focus of the 

 eye-glass ; the cell of which should have a longer screw than usual to 

 admit of adjustment for different eyes. The brass frame is retained 



