VOL. XX.] PHILOSOPHICAL TRANSACTIONS. 269 



in the greater half circuit of the same. Whence Mr. L. computes that 3181 is 

 the number of these eyes that are in both the tunica cornea of a beetle ; if 

 they both make up a whole globe. 



On enlarging the Divisions of the Barometer, in order to measure the Height of 

 the Mercury more exactly. By Mr. Stephen Gray. N" 240, p. 176. 



A, fig. 5, pi. 5, is a long square table, towards one end is erected a square 

 column EB, on which there slides a square socket c, from one side of which 

 proceeds a crooked arm de. At d is a screw hole to receive the screw, and at 

 E a ring to support the tube of the microscope f. From the other side of the 

 socket comes a short arm g, having a screw-hole to receive the long screw 11, 

 whose length may be about 6 or 7 inches ; its lower end, by a small hole in its 

 centre, rests on the end of a small screw, that comes through the screw- hole, 

 in the arm h, which is fixed on the back side of the column ; the upper end of 

 the screw is filed less than the body of the screw, and goes through the centre 

 of the rownd plate without shaking, and to prevent its doing so, either upwards 

 or downwards, there is added a springing plate n, which keeps the shoulder of 

 the screw close to the under side of the plate k ; over this plate goes an index 

 o, and over that a handle l, on the end of the screw which comes through the 

 centre of the plate, which is riveted to the top of the column bb. The teeth 

 of the screw must be of that size as to have just 10 in an inch. The fore side 

 of the column must be divided into inches and tenths, beginning about the 

 height of the socket h, where the lower end of the screw rests, and so conti- 

 nuing to the top of the column. The limb of the round plate must be divided 

 into 100 parts. In the focus of the eye-glass of the microscope is fixed a hair, 

 or very fine silver wire, in a horizontal position. 



To use this instrument, take hold of the handle, and, looking through the 

 microscope, turn the screw till you have brought the hair to touch, as it were, 

 the surface of the mercury m : then observe what divisions are cut on the 

 column by the upper or under edge of the socket, which are tenths of an inch. 

 Observe likewise to what parts the index points on the limb of the round plate, 

 which are hundreds of a tenth, or thousand parts of an inch ; when you per- 

 ceive the mercury varied, raise or depress the microscope, till the hair be brought 

 to its surface, as before ; then by subtracting the less from the greater of the 

 two observed numbers, you will have the variation in inches and thousandth 

 parts. 



This instrument becomes a micrometer on the same principles ; ihe thermo- 

 meter is also capable of the like improvement. 



