530 Dr. Herschel's 'Experiments on the solar, and 



bottom of the long board, towards the other end, where, close to 

 the observer, another pulley is fixed ; and, after going also over 

 this pulley, it returns at the top of the board, to the front of the 

 same vane, to which the other end of it is fastened at the back. 

 By pulling the string either way, the observer may bring forward 

 the moveable vane, or draw it back, at pleasure. 



At the side of the long board is a scale of tens of inches, num- 

 bered from the place of the flame of the lamp, o, 10, 20, 30, 40, 

 and so on to 160. A pair of compasses being applied from the 

 last ten towards the vane, ascertains its distance from the flame, 

 to as great an accuracy as may be required. 



When the transmitting power of a glass is to be tried, it must 

 be placed over one of the holes of the small moveable piece, 

 which then is fastened with a button, upon the opening left for 

 it in the circular wooden screen. Then, looking through the 

 hole of the pasteboard at the two vanes, and bringing that which 

 is seen through the glass near enough to give an image equally 

 bright with that which is seen through the open hole, the obser- 

 vation will be completed. Having measured the odd inches by 

 a pair of compasses, or immediately by a scale, we deduce, as 

 usual, the transmitting power, by taking double the logarithm of 

 the distance of the farthest vane from the lamp, from double 

 the logarithm of the distance of the nearest vane. The remain- 

 ing logarithm is that of the transmitting power, as compared to 

 the light coming directly to the eye from the other vane. 



I have now only to remark, that the use of this instrument 

 requires some practice, especially when coloured glasses are to 

 be examined ; it will, however, be found, that the difference of 

 the colour of the two objects, when their light is brought to an 

 equality, may be overcome by a little abstraction, which is 



