46 G. J. BURCH ON A NEW MICROMETER. 



with the divisions on the stage micrometer, by altering the distance 

 between scale and condenser. But this was a troublesome operation, 

 until I fixed the scale in a tube sliding below the lens — and then the 

 divisions had to be so much smaller, that with my appliances I could 

 not get them accurate. And, besides, it was of no use with large 

 objects or front light, so I gave it up. However, I showed the 

 principle, in a rough way, at a meeting of the Quekett Microscopical 

 Club, on October 25th, 1872, and was somewhat surprised, a few 

 months later, to find, from the Paper on the lC Aerial Stage-Micro- 

 meter," published in the " Monthly Microscopical Journal " of 

 January, 1873, that Dr. Royston-Pigott had been working in the 

 same direction.* Dr. Pigott's paper so completely detailed my 

 principle that I determined to abandon it altogether, and seek " green 

 fields and pastures new." So I looked into the theory of the various 

 forms of Micrometer. 



The early microscopists aimed at getting micrometer and object 

 in the same plane, and so laid the object upon the scale itself, in the 

 lower focus of the objective. The next step was to place a real 

 scale, or spicier lines, in the image at the focus of the eye-lens, 

 that is to say, in the upper focus of the objective. Browning, in 

 his spectroscope, employs the image of a scale in the same place. 

 Then the Aerial Stage-Micrometer reversed the operation with an 

 image of the scale in the plane of the object, formed in the light, 

 before it entered the instrument. In the two foci of the objective 

 all possible variations of real scale or optical image had been used. 

 But every lens has its two conjugate foci. One focus of the eye- 

 piece coincides with the upper one of the objective, the other re- 

 mained. It is only a virtual focus, for its sign is negative. It had 

 not been used, that I know of, except indirectly, for micrometry. I 

 wanted a novelty, so I varied all the conditions, and determined to 

 unite an image of the scale, with the image of the object, after it 

 had left the eye-piece. At first I proposed to use the glass scale and 

 lens of my aerial stage micrometer, with a diagonal tinted- 

 reflector fixed above the eye-piece — like a camera lucida set back- 

 wards. But just then I had occasion to investigate for another pur- 

 pose the theory of the apparent distance of the magnified object. I 

 found that as the focus is altered so the plane of the image varies 



* A micrometer identical with Dr. Pigott's " Aerial Stage-Micrometer " 

 will be found described in " Goring and Pritchard's Micrographia," 1837, p. 

 51. (Ed. Q.M.C.J.) 



