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VII. — On a Microscope ivith Geometric Slides. 

 Bv Keith Lucas. 



[(Read February Yith, 1904.) 



The instrument with which this paper deals represents an attempt 

 to replace the usual planed slides of the Microscope by geometric 

 slides. The application can hardly be considered new, since geo- 

 metric slides are commonly used on measuring Microscopes, yet I 

 am not aware of the existence of any other Microscope suitable 

 for biological work in which such slides are used. 



The arrangement of geometric slide which has been found most 

 suitable for the focussing movements is a tube, concentric with the 

 optic axis of objective and eye-piece, sliding in two V-guides placed 

 near its extreme ends. This arrangement has the advantage of 

 ensuring that rotation of the tube within its V-guides shall not 

 displace the optic axis ; consequently the means adopted for pre- 

 venting this rotation may be of the roughest nature, may, in fact, 

 be only sufficient to prevent such a degree of rotation as would 

 damage the focussing mechanism. Unfortunately the necessity of 

 conforming to the proportions of Microscopes commonly in use 

 has rendered such an arrangement impossible in the case of the 

 slide which carries the condensing lenses. In this case the axis of 

 rotation of the guide-tube has been considerably displaced from the 

 optic axis. 



The photograph reproduced in fig. 49 will serve to indicate the 

 general arrangement of the instrument. 



The main casting, or limb (A, figs. 49-53), is carried further 

 forward than is usual, so as to partially embrace the large body- 

 tube (B, figs. 49, 50, 51), and carries four projections, two at its 

 upper, and two at its lower end, which form the V-guides, in which 

 that tube slides. This slide forms the coarse adjustment. The 

 means adopted to hold the tube against its four guides, and to 

 prevent it from rotating about the optic axis, will be considered later. 

 The large body-tube carries two rings, one (C, figs. 50, 51) inside 

 its upper, and one (C, fig. 51) inside its lower end. Each of these 

 rings has two projections upon it, against which the long narrow 

 inner tube (D, figs. 49, 50, 51) is held. Thus are formed the 

 V-guides of the coarse adjustment. 



The two upper guides of the fine adjustment, and of the coarse 

 adjustment, and the two concentric tubes which slide in these guides, 

 are shown in fig. 50. The substage is carried by a long stout tube 



