168 



Dr. Herschel then, in order to investigate the causes of the spu- 

 rious diameters of celestial objects, relates some observations made 

 upon several of the fixed stars, from which he deduces the following 

 inferences : 



1 . That the spurious diameters of the stars are larger than the 

 real ones, which are too small to be seen. 



2. That the spurious diameters are of different sizes ; but that, 

 under the same circumstances, their dimensions are of a permanent 

 nature. 



3. That the spurious diameters are differently coloured, and that 

 these colours are permanent when circumstances are the same. 



4. That these diameters are proportionally lessened by increasing 

 the magnifying power, and increase when the power is lowered. 



5 . That the above-mentioned increase and decrease is not inversely 

 as the increase and decrease of the magnifying power, but in a much 

 less ratio. 



6. That the magnifying power acts unequally on spurious diame- 

 ters of different magnitudes ; less on the large diameters, and more 

 on the small ones. 



7. That when the aperture of the telescope is lessened, it occa- 

 sions an increase of the spurious diameters ; and when increased, 

 reduces them. 



8. That this increase and decrease is not proportional to the dia- 

 meters of the stars, but that an alteration in the aperture of the te- 

 lescope acts more upon small spurious diameters and less upon large 

 ones. 



9. That stars which are extremely small lose their spurious dia- 

 meters, and become nebulous. 



10. That many causes will influence the apparent diameter of the 

 spurious discs of the stars ; but that, with a proper regard to those 

 causes, the conclusion already drawn, that under the same circum* 

 stances their dimensions are permanent, still remains good. 



A number of experiments then succeed on the spurious diameters 

 of terrestrial objects. The first series of these were made by means 

 of the silver globules already mentioned. The inferences drawn from 

 them are similar to those drawn from the observations of celestial 

 objects, except that the spurious discs of terrestrial objects, contrary 

 to what happens with celestial ones, are smaller than the real discs, 

 and that they are apt to be lost for want of proper illumination, but 

 do not on that account change their magnitude. 



Similar experiments were made with drops of quicksilver, the re- 

 sults of which differed so little from those of the experiments with 

 globules of silver, as not to require any further description of them. 



After two preparatory experiments, one with black and white 

 circles, which showed that no material deception can take place in 

 estimating by such circles, on account of their colour, and another, 

 which showed that no difference in the apparent size of the globules 

 was produced by a different mode of illuminating them in the micro- 

 scope, Dr. Herschel proceeded to measure the spurious disc of one 



