214 PHILOSOPHICAL TRANSACTIONS. [ANNO 1/82. 



vapours. It is difficult to find a criterion of the colours of stars, though I might 

 in general observe that Aldebaran appears red, Lyra white, and so on ; but when 



1 call the stars garnet, red, pale red, pale rose-colour, white inclining to red, 

 white, white inclining to blue, bluish white, blue, greenish, green, dusky, I 

 wish rather to refer to the double stars themselves to explain what is meant by 

 those terms. 



4. The distances of the stars are given several different ways. Those that are 

 estimated by the diameter can hardly be liable to an error of so much as one 

 quarter of a second; but here must be remembered what I have before remarked 

 on the comparative appearance of the diameters of stars in different instruments. 

 Those that are measured by the micrometer, I fear, may be liable to an error of 

 almost a whole second: and if not measured with the utmost care, to near 2". 

 This is however to be understood only of single measures; for the distance of 

 many of them that have been measured very often in the course of 2 years ob- 

 servations, can hardly differ so much as half a second from truth, when a proper 

 mean of all the measures is taken. As I always make the wires of my micro- 

 meter outward tangents to the apparent diameter of the stars, all the measures 

 must be understood to include both their diameters; so that we are to deduct the 



2 semi-diameters of the stars if we would have the distance of their centres. 

 What I have said concerns only the wire micrometers, for my last new micro- 

 meter is of such a construction, that it immediately gives the distance of the 

 centres; and its measures, as far as in a few months I have been able to find out, 

 may be relied on to about T V of a second, when a mean of 3 observations is 

 taken. When I have added inaccurate, we may suspect an error of 3 or 4". 

 Exactly estimated may be taken to be true to about £ part of the whole distance; 

 but only estimated, or about, &c. is in some respect quite undetermined; for it 

 is hardly to be conceived how little we are able to judge of distances when, by 

 constantly changing the powers of the instrument, we are as it were left without 

 any guide at all. I should not forget to add, that the measure of stars, when 

 one is extremely small, must claim a greater indulgence than the rest, on account 

 of the difficulty of seeing the wires when the field of view cannot be sufficiently 

 enlightened. 



5. The angle of position of the stars I have only given with regard to the 

 parallel of declination, to be reduced to that with the ecliptic as occasion may 

 require. The measures always suppose the large star to be the standard, and the 

 situation of the small one is described accordingly. Thus, in fig. 12, ab repre- 

 sents the apparent diurnal motion of a star in the direction of the parallel of de- 

 clination ab; and the small star is said to be south preceding at mn, north pre- 

 ceding at op, south following at qr, and north following at st. The measure of 

 these angles, I believe, may be relied on to 2°, or at most 3°, except when men- 



