5(5 PHILOSOPHICAL TRANSACTIONS, (^ANNO 1780. 



a practice of Hindley's of many years standing, and since followed by myself and 

 others, wherever he made any use of the vernier, to lay the vernier plate in the 

 same plane, or cylindrical surface continued, on which the principal divisions are 

 cut. It is of equal utility, though the vernier be rejected, to lay the index stroke 

 in the plane of the divisions. In this way the divisions being by convenience on 

 the external border of the limb,* 2 sets of divisions are thus rendered incom- 

 modious ; but those that wish 2 sets, as a check, will in a great measure aid 

 themselves, by reading from 2 different parts of the same set of divisions ; which 

 is very easily provided for, by putting an additional stroke on the index plate, at 

 the distance of 9, 11, or any prime number of divisions to IQ, 23, or more ; and 

 reading off from that stroke also ; as before recommended for great quadrants, 

 where the vernier' is proposed to be rejected :-|- so that they will thereby be 

 mutually checked by divisions that had no correspondence in their original for- 

 mation. 



]L A Series of Observations on, and a Discovery of, the Period of the Variation 

 of the Light of the Star marked S by Bayer, near the Head of Cepheus. By 

 John Goodricke^ Esq. Dated York, June IS, 1785. p. 48. 



Mr. Goodricke's first observation was Oct. I9, 1784, and they were continued 

 almost daily, till June 28, 1785. From this series he settled, that the star has 

 a periodical variation of 5^^ S^ 374-"", during which time it undergoes the following 

 changes: 1. It is at its greatest brightness about I day and 13 hours. 2. Its 

 diminution is performed in about 1 day and 18 hours. 3. It is at its greatest 

 .obscuration about 1 day and 12 hours. 4. It increases in about 13 hours. When 

 it is in the first point it appears as a star of between the 4th and 3d magnitude ; 

 but its relative brightness does not seem always to be quite the same, being some- 

 times between ^ and 1, Cephei, and sometimes only equal to, or something less 

 than, I Cephei, or between ^ Cephei and 7 Lacertas. In the third point it ap- 



• It has been 'objected, that laying the divisions on the extreme edge of the limb of the itistm- 

 ment subjects it to injury : but, to obviate this, in a Hadley's quadrant made for me, by my direction, 

 by the late Mr. Morgan, in che year 1756, wherein the vernier is laid even with the divisions, those 

 are protected by a projection of the solid part of the limb, beyond the divisions 5 a rabbet being sunk 

 in the edge of the limb, to clear the vernier. — Orig. 



+ I would not have it thought, from my proposal of rejecting the vernier, that I have any quarrel 

 with it ; I think it a very simple and ingenious contrivance, where it is properly applicable ; that is, 

 where the strokes of the vernier, or their estimated halves, are sufficient for all the precision required 

 or expected from the instrument, as in Hadley's quadrants, theodolites, &:c. : but where still more 

 minute divisions are required than can easily be had by estimation from the vernier ; to do this by a 

 screw, as a supplement to the vernier, appears to me in the light of bringing a more accurate tool to 

 snpply the deficiencies of one less accurate j when the former might, wi)h more propriety, supply the 

 place of the latter altogether, — Orig. 



