ZOOLOGY AND BOTANY, MICEOSCOPY3 ETC. 325 



optical action of an objective, it is necessary to be able to rotate 

 the latter on its axis without using the screw. This is accom- 

 plished by an adapter, the upper part of which screws into the 

 end -of the body-tube, while the lower receives the objective and 

 rotates easily and concentrically on the upper. An object such 

 as the silvered test-plate * is placed on the stage and illuminated by 

 oblique light. The objective is then rotated on its own axis from 

 1/4 to 1/4 or 1/8 to 1/8, and to be approved the sharpness of the 

 image and the character of the spherical and chromatic aberrations 

 must be identical in all positions. If this is not so, there are either 

 defects in the centering or local faults in the lenses or in the cement. 

 This test when used on an appropriate object is a very sensitive and 

 valuable one, and is indeed the only one which furnishes a real test 

 of the centering of the lenses. The other methods which have 

 been recommended for that object test only the centering of the 

 screws, which in regard to the optical action is quite an unimportant 

 matter. 



Cost of Objectives of Large Aperture. — The great increase in the 

 labour necessary to produce an objective of large aperture is but 

 little appreciated by microscopists. Some estimate may, however, 

 be formed from the fact that so low a power as a 1/6 in. of 1'50 N.A. 

 costs 35Z. Even at this price there are no signs of any tendency to 

 underselling or competition among opticians. So far as we have 

 been able to form an opinion, the price in question, large as it is, does 

 not represent more than a very moderate return for the skill which 

 the construction of such an objective requires. 



Finder.f — P. Francotte describes a finder the designer of which 

 is unknown. 



The adapter A (fig. 77), with Society screw, carries a tube B 

 which is kept extended by a spring, but can be pushed back again 

 with very slight pressure. Two screws prevent the tube rotating. 

 To the end of this tube is screwed the conical piece C, 

 the point of which is cut out like a ring punch, so that Fig, 77. 

 when smeared with bitumen or ink it will impress a 

 small circle on the slide about 1/50 in. in diameter. 



When the object is in the centre of the field the 

 objective is removed and replaced by the finder, and 

 a circle is impressed on the cover-glass. The spring 

 prevents any damage to the object. With low powers 

 the circle is readily found again. With high powers 

 the procedure is given as follows, though we should 

 have supposed that even with the highest powers it 

 would not be necessary to have recourse to the finder 

 again : — The finder is replaced on the body-tube 

 without any eye-piece. The piece C is removed and the slide placed 

 so that the circle is in the centre of the opening. A small diaphragm 

 will facilitate this operation. The finder is then removed and the 

 objective screwed on. 



* See this Journal, iii. (1883) p. 125. 



t Bull. Soc. Belg. Micr., xi. (1884) pp. 48-50 



