108 



E. M. NELSON ON EVOLUTION OF THE MICROSCOPE. 



[It may be interesting to note that the screw micrometer for a 

 telescope was invented by WilHam Gascoigne (1639). His plan 

 was to separate the edges of two plates by a differential screw, 

 which had one thread twice the fineness of the other. A frame 

 carrying both plates was moved across the field by the fine screw, 

 the coarse screw moved one plate away from the other at double 

 the speed, therefore both plates moved from each other at an 

 equable rate. Gascoigne, at the age of twenty-four, was killed at 

 the battle of Marston Moor (1644). 



It must be noted that this differential screw, which in 1775 was 

 applied by Ramsden to a divided lens micrometer, was not used 

 as a means of obtaining a slow movement with two coarse threads, 



Fit. 18. 



but only for opening or closing at a uniform rate the edges of the 

 micrometer plates. If at the present time a similar apparatus 

 were required, it might be accomplished by means of a right and 

 left handed screw on the same pinion. 



Fine silver wires were substituted for the edges of the plates 

 about 1666, and subsequently Troughton replaced the wire with 

 a spider's web.] 



To return, however, to our legitimate subject, it was at this 

 time (1738) that Dr. Lieberkuhn introduced the reflector, which 

 is still called after him ; the design can hardly be called original, 

 as it had been figured and described by Descartes one hundred 

 years previously (.see Part 1, Fig. 2, p.. 350). To show the crude 

 form in which it was first applied. Fig. 18, which was found by 



