of ascertaining the Magnitude of small celestial Bodies . 43 
Aquilae. When y was perfectly distinct and round, the ex- 
tremely small stars were dusky and ill defined ; the excessively 
small ones were still less defined. As there are stars of all sizes 
in this neighbourhood, I saw some so very minute, that they 
only had the appearance of a small dusky spot, approaching to 
mere nebulosity. By very long attention I perceived many small 
dusky nebulous spots, which had it not been for this attention 
might have been in the field of view without the least suspicion. 
( 10. ) From this we find that stars, when they are extremely 
small, lose their spurious diameters, and become nebulous. 
July 7, 1780. I saw the spurious diameter of Arcturus gra- 
dually diminished by a haziness of the atmosphere till it 
vanished intirely. 
A more circumstantial account of this observation has alread} r 
been given ; and some other causes that affect the spurious 
diameter of the stars, have been pointed out in the same paper, 
such as tremulous air, wind, and hoar-frost.* 
January 31, 1783. The star in the back of Columba makes 
a spectrum, about 5 or 6 " long, and about 2" broad, finely 
coloured by the prismatic power of the atmosphere at this 
altitude. 
July 28, 1783. Fomalhaut gives a beautiful prismatic spec- 
trum, on account of its low situation. 
July 17, 1781. With a new lens, power between 5 and 6 
hundred, I saw £ Aquarii, and found the vacancy between the 
two stars exactly 2 diameters. With my old one, power only 
460, it was full 2 diameters. As it should have been larger 
with the high power than with the low one, it shows that the 
best eye-lens will give the least spurious diameter. 
* See Phil. Trans, for 1803, page 224. 
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