Limit to Divining-jpoiccr. By J. J. Lister. 35 



circular holes were pierced in brass of different diameters, and 

 various objects were viewed through them. Venus being at th;U 

 time (December 1831) a crescent and a brilliant morning star, 

 she was seen with the whole pupil as an indefinite luminous object 

 with sliort rays; through an aperture 0-005 in. in diameter, slie 

 was imperfectly defined and with some radiation ; through 003, 

 round, witli a little surrounding light; throu-di 0*040 and 0*042 

 a very well-defined small round spot, like a star in a fine telescope ; 

 through * 034, the same kind of spot, rather larger, and it con- 

 tinued to increase in size as the aperture was diminished. 



2. Two candles were placed in a dark room 6 in. apart ; be- 

 yond them, at 1 foot from each, was set the ball of a thermometer 

 0'36 in. in diameter; and the candles being screened from the 

 eye, their image in the ball was viewed at 3 feet distance. 



With the whole pupil it seemed to be double but had no 

 defined interval, and through 0* 095 in. it was much the same. 

 Through 0*063 the division was more plain but not well marked ; 

 through • 049 gave two round spots about 1 diameter apart ; 

 through • 042 a very well defined double star, ditto ; through 

 0*034 the same, with the spots rather larger ; through 0*027 they 

 were but just separated ; through 0*021 they ran together. 



In both these trials the object was seen best through the holes 

 from * 049 to * 034 in., yet on varying the second experiment 

 by placing white paper behind the ball the image was more dis- 

 tinct viewed with the whole pupil ; and green glass interposed 

 showed it much sharper than before through the larger aperture. 



3. As another example, two square holes were made in black 

 paper, the sides of each 0*1 in. and their centres 0*4 in. apart; 

 both were covered with ground glass. The paper was fixed so as 

 to intercept the light from the sky, and the image of the holes, 

 reflected from a globule of mercury, was received into an achro- 

 matic Microscope. 



They were then seen as two equal round spangles on a black 

 ground, with an interval of J^ of a diameter between them. 



On covering the ground glass with gauze-paper, the round spots 

 were so much diminished as to be a full diameter of each apart. 



When the gauze-paper was withdrawn from one hole, the spot 

 which represented that hole became much larger as well as brighter, 

 the other showing moj-e faintly than before ; and when, one hole 

 being covered with ground glass, the direct rays of the sun were 

 admitted through the other, its image wholly overspread and 

 obliterated that of the first by its greater splendour. 



Both holes being again covered with ground glass, the aperture 

 of the object glass of the Microscope which had been 0*44 in. 

 was reduced to 0*15 in., and the image of the two squares then 

 became united into one oblong spot. 



In the instances given we see that pencils received direct from 



D 2 



