30 PHILOSOPHICAL TRANSACTIONS. [ANNO 1781. 



the place of the lens a. The whole instrument may be furnished with a handle, 

 as at m ; or, if larger, it may have 3 feet to stand on a table. 



On the last principle it must be owned, the common screw has the advantage, 

 as 2 screws will produce more friction than one; and besides, in the compound 

 engine there is an additional friction from the piece fg (fig. 2) on the pillars be- 

 tween which it moves. 



Another case in which this machine may be employed, is in the micrometer. 

 Thus, let the screw ab (fig. 4) of 50 threads in an inch be turned round by the 

 index c, which moves on the graduated circle ecd in the direction cd. Within 

 the screw ab is the smaller one ap of 51 threads in an inch, retained from moving 

 round by the bar gfh. The piece af is continued to k, where it forms a fine 

 point. To use the instrument, let it be adjusted to the telescope or microscope 

 by which you are to view a star, or some small object, and let the point k appear 

 just to touch one edge of the object. Then turn the index c, and the point k 

 will advance upwards till it appears to cover the other edge of the object, and 

 thus you can determine its size. The point k will advance at each complete 

 turn of the index ^Vo- °* an nicn 5 anc * '*" tlie circle be divided into 80 equal 

 parts, 1 of which, if it is an inch in diameter, will be very observable, while the 

 index moves over one of these, the point k will advance ,, „ ,'„ „ „ of an inch. 



Thus, for example, suppose I am to measure the diameter of a nervous fibre 

 in the medullary substance of the brain, I make the point k appear close to one 

 edge, and turn the index till the same point pass over the fibre, and appear to 

 touch the other edge: I then look on the graduated circle ecd, and perceive that 

 the index c has passed over, suppose, 23.2 divisions. Hence I conclude the dia- 

 meter of the fibre to be 23.2 X wAinr = Trnnr °f an » n ch, which is nearly the 

 size as found by the accurate observations of Dr. Monro. There should be a 

 Nonius's scale on the index which will measure to -fc of a division. 



As the index c must continue close to the plate ecd, while at the same time 

 it turns round the screw ab, which is continually rising, it must be made as in 

 fig. 5, where a, b, are 2 small pieces which play in a groove in the screw ab (fig. 

 4) while the groove cd (fig. 5) in the index is filled up by a protuberance of the 

 plate ecd (fig. 4); the piece below the groove cd (fig. 5) being sunk into that 

 plate. The whole machinery may be inclosed in a cylinder of brass reaching 

 from b to l (fig. 4), so that the point of the screw kl may be without it, and 

 the sides of the cylinder may be open at ecd. 



It is further to be observed, that what has been said goes on the supposition 

 that the point k, in the micrometer, is equally magnified with the object we are 

 to measure. But, if this point be placed in the focus of the eye-glass of a double 

 microscope; when it moves it will pass over, not the object itself, but its image, 



