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of the globules. For this purpose, he viewed it with apertures of 

 different dimensions and of different kinds. Some of them being of 

 the usual annular opening or outside rim, from 6'5 to 8*8 inches, 

 which reflected less than half the light of the mirror, produced a spu- 

 rious disc less than '18 in diameter; while the whole light of the . 

 mirror gave a disc of '31 : he thinks it fair to conclude, that it is not 

 the quantity of light, but the part of the mirror from which it is re- 

 flected, that we are to look upon as the cause of the magnitude of 

 the spurious discs of objects ; and this, he says, points out an im- 

 proved method of putting any terrestrial disc, that we suspect to be 

 spurious, to the test : for the inside rays of a mirror will increase the 

 diameter of those discs ; but the outside rays alone will have a greater 

 effect in reducing it, than when the inside rays are left to join with 

 them. 



Dr. Herschel then placed two other globules at a small distance 

 from each other, without having previously measured either their 

 size or the distance between them. Upon viewing them with a power 

 of 522'7, they appeared in the shape of half-moons ; he estimated the 

 vacancy between the cusps to be one fourth the diameter of the 

 largest ; and found afterwards, on measuring the diameters and di- 

 stance, that his estimation did not differ -5 forth of an inch from the 

 truth. In a second experiment the difference between the real and 

 the estimated distance was still less. 



In order to ascertain whether these half-moons were real or spu- 

 rious, Dr. Herschel viewed them first with the inside rays of the 

 mirror, then with the outside rays, and lastly, with the whole mirror 

 open, but no alteration in the distance of the lunes could be per- 

 ceived. He then divided the aperture of the mirror into two parts, 

 one from to 4'4 inches, the other from 4 '4 to 8- 8 ; and found, on 

 measuring the spurious diameter of a globule, that with the inside 

 rays it was -40 ; with the whole mirror open it was '31 ; and with 

 the outside rays it was -22. 



From this, he says, we may conclude, that the diameters given by 

 the inside rays, by all the mirror open, and by the outside rays, are 

 in an arithmetical progression ; and that the inside rays will nearly 

 double the diameter given by the outside. 



a Lyrse being then examined in the same manner, its spurious disc 

 was found to be small with the outside rays ; with the whole mirror 

 open it was larger ; and with the inside rays it was largest. 



The double star a Geminorum was then viewed with a power of 

 410*5 ; with the outside rays they appeared unequal, and 1^ diameter 

 of the largest asunder ; with the whole mirror open they were more 

 unequal, and 1^ diameter of the largest asunder; with the inside 

 rays they were very unequal, and If of the largest asunder. 



The foregoing experiments show, the author says, that if it had 

 not been known that the apparent discs of the stars were spurious, 

 the application of the improved criterion of the aperture would have 

 discovered them to be so ; and that, consequently, the same improve- 

 ment is perfectly applicable to celestial objects. 



