74 SECOND-CLASS MICROSCOPES. 



by a wire spring that bears against its under surface, as to be easily 

 moved by either or both hands ; and as access can be readily 

 gained to this spring by detaching the plate T from the stage, it 

 may either be removed altogether so as to leave the stage free, or 

 may be adjusted to any degree of stiffness desired by the observer. 

 The Object-holder has a ledge v for the support of the slide ; and 

 it is also provided with a small spring w, attached to it by a 

 milled-head, by turning which the spring may be brought to bear 

 with any required pressure against the edge of the slide laid upon 

 the object- holder, so as to prevent it from shifting its place when 

 rotation is given to the stage, or when, the instrument being 

 placed in the horizontal position, the stage becomes vertical. The 

 central Tube of the Stage is adapted to receive fittings of various 

 kinds, such as Diaphragm-plate, Dark-well, Paraboloid, and Pola- 

 rizing prism ; and it can also carry either a Webster Condenser or an 

 ordinary Achromatic Condenser. This instrument may be furnished 

 either with First-class or with Second-class Objectives ; the latter 

 are well adapted for Educational use ; but the Scientific investigator 

 will do well to provide himself with the former, bearing in 

 mind, however, the caution already given (§ 30) as to Angle of 

 Aperture.* 



51. Collins' s Harley Binocular. — This instrument, represented 

 in Fig. 39, deserves mention, not merely for its general excellence, 

 but also on account of certain special adaptations which render it 

 peculiarly convenient to the Medical Student. It is substantially 

 framed and well hung on the Ross model ; and can be furnished 

 with all the Accessories usually needed. The caps of the Eye-pieces 

 are provided with shades, which cut off the outside lights from each 

 eye ; these can be adapted to any instrument, and the Author can 

 speak strongly of their value from his own experience. The 

 "Wen ham prism at the common base of the bodies is fitted into an 

 oblong box, which slides through the arm that carries them ; this 

 contains, in addition, a Nicol analyzing prism, and is also pierced 

 with a vacant Aperture ; so that by merely sliding this box trans- 

 versely until the Aperture comes into the axis, the instrument 

 may be used as an ordinary Monocular ; or, if the analyzing 

 prism is made to take the place of the Wenham, whilst the 

 polarizing prism beneath the stage is brought into position by 

 rotating the Diaphragm-plate in which it is fixed, it is at once 

 converted into a Polarizing Microscope. The Objectives screw into 

 another movable fitting which holds two at once, and slides 

 in the back and front direction, so that either of the two powers 

 can be exchanged for the other with the greatest facility. The 



* Thus the small-angled 4-10ths Objective of Messrs. Smith and Beck 

 is much better adapted to Binocular use than the large-angled 4-lOths 

 of the same makers. On the other hand, as the l-4th inch Objective is 

 unsuited to Binocular use, the choice between a wide and a narrow 

 angle will have to be determined by other considerations v § 131.) 



