Mr. Troughton on dividing Instruments. 129 
It might have been mentioned sooner, that in the instance 
which I have selected as an example of my dividing, the opera- 
tion took place when the season of the year, and the smoke of 
London, had reduced the day to scarcely six hours of effective 
light ; and rather than confine my labours within such narrow 
limits, I determined to shut out the day-light altogether. 
Fig. 7 shews the construction of the lanterns which I used. 
A very small wick gave sufficient light, when kept from di- 
verging by a convex lens ; while the inclining nossel was 
directed down exactly upon the part looked at, and the light, 
having also passed through a thin slice of ivory, was divested 
of all glare. I enter into this description, because, I think, I 
never saw my work better, nor entirely to so much advan- 
tage as in this instance ; owing, perhaps, to the surrounding 
darkness allowing the pupil of the eye to keep itself more ex- 
panded, than when indirect rays are suffered to enter it. The 
heat from a pair of these lanterns was very inconsiderable, 
and chiefly conducted along with the smoke up the reclining 
chimney. 
Previous to cutting the divisions, the parts now described 
must be adjusted. The cutting apparatus must be placed with 
the dividing point exactly at the place where the first line is 
intended to be drawn, and clamped, so that the adjusting screw 
may be able to run it through a whole interval. The micro- 
scope H must be firmly fixed by its two pillars b b to the main 
frame, with its micrometer head at zero; and with its only 
wire in the line of the radius, bisecting the first of the 256 
dots. And it should be observed, that the cutting frame and 
this must not vary respecting each other, during the time that 
the divisions are cut; for any motion that took place in either 
MDCCCIX. S 
