IMPROVED REFLECTING CIRCLE. 



has a flanch on the upper end, by which it is screwed to the 

 index H, and upon this, the under glass G, fig. 1, is fastened, 

 by other screws passing through a piece projecting from the 

 back of it. The axis is held in its place by a collet r, fitted 

 on a square part of it, and held fast by a screw s j beneath this a 

 piece is fixed on in the centre of the circle, the edge of its flanch 

 being shown by t in fig. 1; it is part of the screw which holds 

 on the spring socket K, for the handle I. The upper end of 

 the centre piece N, which comes up above the circle, is turned 

 extremely true, and upon this the index B is fitted, or rather a 

 brass ring v screwed to it, so as to turn round upon it as a 

 centre. 



The telescope C is fixed to the index by two cocks and 

 by two screws XX, in these it can be raised up or lowered, 

 to adjust the different brightness of the two objects seen in the 

 horizon glass D, the one reflected from the central mirror G, 

 and the other seen directly through it. The dark glasses at E 

 are intended to moderate the light of the sun, in passing from 

 the index to the horizon glasses ; the frames containing these 

 glasses have holes E through them, to sefe through the telescope 

 and horizon glass j the other dark glasses, F, are situate behind 

 the horizon glass D, and may be turned up or down, as occasion 

 requires. 



The instrument is used in the same manner as the common 

 reflecting cii^le j the angle being first taken on one side of 

 the parallelism of the glasses, and then on the other j so that 

 the angle is doubled j then it is repeated on a fresh part of the 

 circle, as many times as the observer thinks proper, and the 

 product divided by the number of observations taken. The 

 mode of taking these observations is explained at full in Dr. 

 Rees's Cyclopaedia, and in Dr. Mackay's publication on the 

 means of finding tke longitude. 



VI. 



117 



