238 MEMORANDA. 



the lever D, the longer one,^, has a hole, to allow the screw, 

 h, to pass without touching it. 



g, a screw, holding the two springs and the elastic arm to 

 the arm of the cap. 



h, a milled screw, to adjust the elastic arm C, so as to 

 bring the diamond point away from the centre, according to 

 the size of the ring required. 



I have one attached to a half-inch, but the quarter is by 

 far the most useful, as this is the power I generally employ 

 in searching any new material. 



Before commencing the examination of a slide, the latter 

 should be firmly fixed to the stage, by bringing the slip to 

 press tightly on its edge. Having now found any particular 

 specimen and brought it into the centre of the field, and 

 having been careful to adjust the mirror in a line with the 

 tube, if not using a condenser, the body of the instrument 

 may be run up and the cap A slipped on to the end of the 

 object-glass, with the upright arm, B, either directly in front, 

 or behind, in a line with the stem. The whole may then be 

 moved down again till the scratching-point touches the surface 

 of the cover, which can easily be seen by the movement of 

 the lever when looking at it horizontally, and applying the 

 finger to the side of the screw h, the cap may be turned 

 round on its centre, making a neatly-turned circular scratch 

 on the cover, with the object perfectly central. By working 

 the slide upwards or downwards, and making a straight line 

 at the side, either up to the e(\^e itself, or crossing a longer 

 line parallel with the edge, and produced by using the other 

 movement of the stage, any particular circle may always be 

 found at once, and may also be registered on the end of the 

 slide. After the circle has been completed, the vertical 

 motion of the stage will produce a line with a loop at the endy 

 which is, perhaps, the most ready guide to the object sought 

 for. 



After a number of trials and various alterations, the present 

 arrangement is the simplest and most effective 1 have been 

 able to devise; but being only an amateur workman, it does 

 not contain so many " perfections," perhaps, as any of our 

 celebrated opticians may be able to add to it should it become 

 an article of " manufacture ;" as, for instance, a wheel and 

 pinion to give the circular motion, and again, by a graduated 

 scale on the lever D, the size of the circle, which ranges up 

 to an eighth of an inch, might be determined beforehand with 

 the greatest nicety. Slides tlius marked are by no means 

 conspicuous, and require to be seen by reflected light to 

 detect the rings. Their appearance may be judged of by the 



