CROUCH, AND HOw's STAND. 11] 



ance witli good workmanship in the construction of the 

 stand. 



The chief point of novelty is the fine-adjustment, 

 shown in detail in fig. 63. The solid bar A, carrying 

 the optical body B, is suspended on the front ends of 

 the two broad, flat, parallel tempered-steel springs c c, 

 the other ends of which are attached to the limb D. 

 The pressure of the focussing screw E, by the point at 

 F on the solid bar, forces down this bar, the springs 

 bending sufficiently to allow about J-in. range of motion 

 Downwards from the normal position. The actual motion 

 of focussing displaces the optic axis slightly ; but this 

 displacement is attended with no inconvenience, except 

 where the microscope is provided with a rotating stage. 

 This mode of focussing must be regarded as practically 

 free from friction, as there are no metal surfaces in 

 contact ; the only friction is between the point of the 

 screw at F, where it acts on the bar by pressure. The 

 suspension of the optical body is strictly on the two 

 springs c C. 1 



How's (Farringdon Street) Student's instrument (fig. 

 64) is deserving of a place among microscopes designed 

 for general use. The stand is of brass, firm and well 

 finished ; the body is fitted with coarse and fine adjust- 

 ments for focussing; and a draw- tube for increasing 

 the magnifying power of the eye-piece. The stage 

 has an arrangement, simple but novel in construction, 

 by which a near approach to a universal movement is 

 obtained. The movable, or upper plate, is held to the 

 fixed lower plate with springs, and, although offering 

 a convenient resistance, allows of a smoothness of 

 motion quite remarkable. It resembles the magnetic 

 stage, but is far more reliable, and can be moved up- 

 wards, downwards, laterally, or in a slanting direction, 

 thus enabling the microscopist to follow living objects 

 with great facility, superseding to some extent the 

 more expensive mechanical stage. A dividing set of 

 object-glasses is supplied with the B eye-piece, thus 

 giving a range of power varying from 40 to about 200 

 diameters. 



(1) Journal R. M. S , p. Ill 1S61. 



