1870.] 



Physics. 



565 



a sheet of gelatine, such as is used for tracing by engravers, was 

 securely fixed over an engraving, and with a sharp steel point 

 (made by grinding down the end of a small round file), the lines 

 of the original traced pretty deeply on the transparent substance. 

 Lead-pencil or crayon dust was then lightly rubbed in with the 

 finger, and the picture was at once ready for use. A number of 

 such drawings could be easily carried between the leaves of a book, 

 each in succession being held in a frame or cell made of two plates 

 of glass separated by a frame of thin card or three edges, and united 

 by paper or muslin pasted around the same edges. The effect of 

 these drawings in the lantern was excellent, and their ease of pro- 

 duction very great. 



A most valuable adjunct to the microscope has been made by 

 Mr. J. Zentmayer. It is a mechanical finger, which in the study 

 of diatoms forms a substitute for the clumsy fingers of the human 

 hand, to do the delicate work of picking up rare and valuable dia- 

 toms detected by the microscope, and to transfer them to a slide for 

 preservation. The instrument may be readily understood on reference 

 to the accompanying cut. 



a is the top plate of 

 the mechanical stage ; the 

 circular plate is omitted. 

 The cap b is fitted to the 

 lower body below the stage, 

 into which cap the new 

 sub-stage c is fastened by a 

 narrow tube, wide enough flm F\A 

 to admit illumination from 

 the mirror. As the lower 

 body is movable up and 

 down by a rack, another 

 movement is gained which 

 is necessary to accomplish the result. The difference of the size 

 of the aperture of the stage and the diameter of the tube which 

 connects the sub-stage with the cap a is equal to the movement of 

 the mechanical stage, and this is found more than sufficient, d is 

 the clamp by which the finger is attached to the stage by means of 

 the screw e. A steel cylinder, g, is nicely fitted into the top and 

 bottom of the tube f, leaving room inside for a light spring to press 

 the steel cylinder upwards. To prevent turning, the spring j is 

 provided at h with a steel pin, accurately fitted into the fork at the 

 top of the tube f. By turning the nut k the spring j is elevated 

 and depressed, giving nice adjustment to the needle n in case the 

 finger is to be attached to a microscope not having rack movement 

 to the cap b, to bring the end of the hair and the object in close 

 approximation. The end of the spring j forms a little ring, with 



