166 On dhmling Insiniments, 



within such narrow liniils, I deterinint-d to shut ont the 

 day-light altogether. Fig. 7. shows the construction of the 

 ]anteln^ which I used. A very small wick gave sufficient 

 lislht, when kept from diverging bv a convex lens ; while 

 the inclining no?sel was directed down exactly upon the part 

 looked at, and tlie light, having also passed through a thin 

 slice of ivory, was divested of all iilare. I enter uito this 

 description, because, I think, 1 neversawmy work lietter, 

 nor tntirc'y to so nnich advantage as in this instance; 

 owius?, perhans, to the surrounding darkness allowing the 

 pupil of the eve to keep itself nnore expanded, than when 

 indirect rays, are suflered to enter it. The heat trom a pair 

 nf these lanterns was very inconsiderable, and chiefly con- 

 ducted 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 ad- 

 justing screw mav be able to run it through a w hole inter- 

 val. 'J'he microscope H must be firmly fixed by its two 

 ■pillars lb \o the main frame, with its micrometer head at 

 zero; and with us onlv wire in the line of the radius, bi- 

 secting the first of the 256 dots. And it should be observed, 

 that the cutting frame and ibis must not vary respecting 

 each other, during the time that the divisions are cut ; for 

 any motion that took place in either would go undiminished 

 to the account oi error. The microscope I is also fastened 

 to the main frame ; but it is only required to keep its posi- 

 tion unvaried, while the divisions of the sector pass once 

 under its notice 3 for it must have its wire adjusted afresh 

 to these divisions at every distinct course. The microscope 

 I has two wires, crossing each other at an angle of about 

 40"'; and these are to be placed so as to make equal angles 

 with the divisions of the sector, which are not dots, but 

 lines. The sectorial arc must also be adjusted to its proper 

 radius by the screw E, Fig. 5 ; i. e. while the main frame 

 has been carried alonsj the circle through a mean interval 

 shown by H, the sector must have moved through exactly 

 l6|^ths of its divisions, as indicated by I*. 



Things 



* For the sake of simplicity, the ncrnunt of ihe process is carried on as if 

 the roller measured the mean interval without error: But it was said 

 (page 84) that the roller, in a continued motion quite round the circle, would 

 in some part ot its cmirse err by 30" or more; therefore, when that is the 

 fase, an extreme run of the roller cannot agree with a mean interval of the 



30" 

 circle nearer than -r^ — 0'-23 ; and most probably this kind of error wiH 



