144 



Transactions of the Society. 



terminated in a drum-head, two threads were cut, one being twice 

 the pitch of the other ; if, for example, one screw gave one milli- 

 metre the other would give half a millimetre of movement for each 

 complete revolution of the drum-head. There were two separate 

 sliding plates, one, which we will call A, carried the other B, with 

 it. ^ The slow-speed screw was connected with the sliding plate A 

 and moved it, together with the plate B, in a certain direction, at the 

 rate of half a millimetre for a revolution, but the other screw was 

 attached to the plate B, and moved it in an opposite direction at 

 double the speed; the resultant of these motions being that the 

 plates were either separated from, or brought to, a certain point at 

 a uniform speed of half a millimetre for each revolution of the drum- 

 head. 



A similar action is now obtained by means of right and left- 

 handed screws, but as such things were quite unknown in Gascoigne's 

 time, one cannot help admiring the ingenuity of his invention. 



Each plate carries the half of a biconvex lens, and by this simple 

 -device measurements of small intervals can be made with great 

 accuracy ; so also the diameter of the disc of light, seen in front of 



Fig. 34. 



Fig. 35. 



the eye-piece of either a telescope or a Microscope, commonly known 

 as the Ramsden disc, can be readily determined. 



This ingenious system of micrometry by means of a divided lens 

 or two lenses is due to Savary in 1743, but Dollond in 1754 effected 

 a substantial improvement by makiL 2; the bnses move in the line of 

 their section. 



It appears then that Gascoigne invented the screw movement, and 

 Dollond the divided lens, so it is not easy to see where Ramsden 

 comes in ; but he may have been, and probably was, the first to apply 

 this form of micrometer to the measurement of the diameter of the 

 emergent pencil of a telescope, for the purpose of finding its power ; 

 at any rate he made micrometers on this principle, and they were 

 known as " Ramsden's Dynameters." These instruments are still 

 made and sold by opticians who keep accessories for the telescope, but 

 they are now called dynamometers. 



In fig. 34 complete revolutions of the drum-head are indicated on 



