167 



determine the first- mentioned question, made a series of experiments, 

 of which the following is a summary account. 



Dr. Herschel's first experiment was made upon the heads of pins, 

 the size of the largest of which was '1375, and that of the smallest 

 0425 of an inch. These pins were placed at the distance of 2407 '85 

 inches from the centre of the object-mirror of the author's ten-feet 

 telescope ; the focal length of the mirror of which, on Arcturus, is 

 119-64 inches, but on the objects above mentioned 125-9. 



Dr. Herschel soon found that none but the smallest of those objects 

 could, at the distance at which they were placed, be of any use ; and 

 that an object of that size, namely, '0425 of an inch, may be easily 

 seen, in his telescope, to be a round body when the magnified angle 

 under which it appears is 2' 18"' 9, and that with a high power a 

 part of it, subtending an angle of 0"'364, may be conveniently per- 

 ceived. 



In the second experiment, Dr. Herschel made use of globules of 

 sealing-wax. These globules were of different sizes, from -0466 to 

 00763 : and the result of the experiment was, that, with a globule 

 so small as -00763 of an inch of a substance not reflecting much light, 

 the magnified angle must be between 4 and 5 minutes before we can 

 perceive it to be round. But it also appears that a telescope, with 

 a sufficient power, will show the disc of a faint object when the 

 angle it subtends, at the naked eye, is no more than 0"'653. 



The third experiment was made with globules of silver, formed by 

 running the ends of very fine silver wires into the flame of a candle. 

 The size of these globules was from -03956 to -00556, and the di- 

 stance of these objects from the mirror of the telescope was increased 

 to 2370*5 inches. By this experiment it was found that the telescope 

 acted very well with a high power, and would show an object, sub- 

 tending only 0"-484, so large, that it might be divided into quarters 

 of its diameter. 



The fourth experiment was made with globules of pitch, bees' wax, 

 and brimstone. From the two first-mentioned substances, no satis- 

 factory inference could be drawn ; but when four globules of brim- 

 stone, the sizes of which were '00962, -009125, '00475, -002375, 

 were viewed with a power of 522- 7, the three first appeared round ; 

 the fourth was invisible till a dark blue paper was placed a few inches 

 behind it. The angle it subtended was 0"'207. 



The fifth experiment consisted in observing the globules of sealing- 

 wax and those of silver at a distance still greater, namely, 9620-4 

 inches, with a power of 502 : the smallest globules of the former 

 substance were invisible at that distance ; but all the silver globules 

 except the smallest (which, having met with an accident, could not 

 be examined,) appeared round. 



The sixth experiment was made by fixing some of the silver glo- 

 bules on a post, and illuminating them, by holding a lantern against 

 them : with a power of 522- 7 they were all seen perfectly well, but 

 the light thrown on them was not sufficient to allow of making an- 

 gular experiments upon them. 



