364 PHILOSOPHICAL TRANSACTIONS. [aNNO 1794. 



these holes, it is easy, by taking hold of the ends of them which project below 

 the bottom of the box, to turn about the cylinders on their axes, even without 

 opening the box. It was said above, that the height of the vertical wing attached 

 to each of the cylinders was equal to the height of the cylinder itself: — this must 

 be understood to mean, not the total length of the cylinder, comprehending 

 that part of it which passes into, and through the bottom of the box ; but merely 

 its height above the bottom of the box, or part projecting, namely 1^ inches. 



As it is absolutely necessary that the cylinders should constantly remain pre- 

 cisely perpendicular to the bottom of the box, or parallel to each other, it will 

 be best to construct them of brass, and instead of fixing them immediately to 

 the bottom of the box (which being of wood may warp), to fix them to a strong 

 thick piece of well hammered plate brass, which plate may be afterwards fastened 

 to the bottom of the box by means of one strong screw. In this manner 1 of 

 his best instruments are constructed. And, in order to secure the cylinders 

 still more firmly in their vertical positions, they are furnished with broad fiat 

 rings, or projections, where they rest on the brass plate ; which rings are -J^ of 

 an inch thick, and equal in diameter to the projection of the wing of the cylin- 

 der, to the bottom of which they afford a firm support. These cylinders are 

 also forcibly pulled against the brass plate on which they rest, by means of com- 

 pressed spiral springs, placed between the under side of that plate, and the lower 

 ends of the cylinders. 



Of whatever material the cylinders be constructed, and whatever be their 

 forms or dimensions, it is absolutely necessary that they, as well as every other 

 part of the photometer, except the field, should be well painted of a deep black 

 dead colour ; to prevent the inconveniencies which would otherwise arise from 

 reflected light, and from the presence of too great a number of visible objects. 

 In order to move the lights to and from the photometer with greater ease and 

 precision, he provided 2 long and narrow, but very strong and steady tables, in 

 the middle of each of which there is a straight groove, in which a sliding car- 

 riage, on which the light is placed, is drawn along by means of a cord fastened 

 to it before and behind, and which passing over pullies at each end of the table, 

 goes round a cylinder, which is furnished with a winch, and is so placed near 

 the end of the table adjoining the photometer, that the observer can turn it about, 

 without taking his eye from the field of the instrument. Many advantages are 

 derived from this arrangement ; as first, the observer can move the lights as he 

 finds necessary, without the help of an assistant, and even without removing his 

 eye from the shadows; 2dly, each light is always precisely in the line of direction 

 in which it ought to be, in order that the shadows may be in contact in the 

 middle of the vertical plane of the photometer ; and 3diy, the sliding motion of 



