210 USE OF THE MICROSCOPE. 



President of the Royal Society, described a plan of Cuff's, oi 

 placing a lattice of fine silver, or other wires, in the focus of 

 the eye-glass of the compound body, the individual wires being 

 distant from each other -^-^ of an inch, these were crossed at 

 right angles by others at the same distance apart, and so 

 contrived as to divide the whole area of the field of view into 

 squares, whose sides were just -^^ of an inch in length, and 

 as the image of any object under examination is formed where 

 the micrometer is placed, it follows that such image may be 

 readily measured : this appears to have been the first applica- 

 tion of a scale to a magnified image. 



Benjamin Martin also, about the same time,* contrived a 

 micrometer for his compound microscopes: this consisted of 

 a screw having fifty threads in the inch, and made to revolve 

 in the focus of the eye-glass; one end of the screw was 

 pointed, and the other was provided with a hand, or index, 

 which could be turned upon a dial, like that of a watch, 

 whose circumference was divided into twenty parts, the 

 value of each division therefore was -joVo o^ ^^ inch. The 

 object to be measured was placed on the stage in the usual 

 manner, and was so adjusted that the image of one of its sides 

 should be, as it were, applied to the point of the screw when 

 the hand of the index was at zero ; the number of revolutions 

 and parts of the same that may be made during the passage 

 of the point of the screw to the opposite side of the object 

 would give its dimensions. 



The elder Adams employed an instrument of the same 

 kind, which was clamped by a screw to the outside of the 

 compound body, and a needle, acted on by a screw with fifty 

 threads to the inch, was passed through a small hole in the 

 side of the body, so as to be in the focus of the eye-glass ; 

 the value of each turn of the screw was known by an index, 

 which pointed to a series of divisions on a circular plate fixed 

 at right angles to the axis of the screw. To ascertain with 

 ease a small part of an inch, a sectoral scale was contrived ; 

 this consisted of two lines, which formed with each other an 



* Micrngraphia Nova, p. 10. 



