Parallaxes of ^ and Cassiopeise. 7 



stancy in the focal zero point ; and I shall therefore adopt the tem- 

 perature coefiBcient from the second solution, viz: 



— 0.000424 =b 0.000027 



This coefficient holds good for a distance of 100 divisions of the 

 scale. For one division of the scale it will be : 



— o'' 00000424 db 0.00000027, or — o".oooi 19 =ti o".ooooo8 . 



The evidence as to the reality of this temperature coefficient seems 

 to be very strong, notv^ithstanding that the scale values obtained 

 for the Pleiades plates* did not appear to vary with temperature. 

 In the light of the present evidence we may perhaps be justified in 

 ascribing this to the comparatively small changes of temperature 

 throughout the Pleiades series, and to the fact that the last two 

 Pleiades plates furnish a very discordant scale value, which tends 

 to conceal the smaller temperature effects. No satisfactory explana- 

 tion of this latter circumstance suggests itself, unless we assume 

 that the glass scale had been removed temporarily from the measur- 

 ing machine; and that when replaced, it made a small angle Avith 

 its former position. It seems best, therefore, to disregard the last 

 two Pleiades plates in deducing a definitive scale value. If we 

 do this, the Pleiades series give for the mean scale value 28". 01 38, 

 corresponding to a mean temperature of the telescope 41°. 6, and a 

 mean focal reading 7.88. For a plate having any other tempera- 

 ture {t) and focal reading (/), we ought therefore to use a scale 

 value computed by the following formula: 



Scale value = 2S".oi38 — o".oo90 (/ — 7.88) — o".oooii9(< — 41.6) («) 



On the other hand, if we prefer to retain the scale values from 

 the last two Pleiades plates, we should have : 



Scale value = 28".oi24 — o".oogo(f — 7.87) — o''.oooii9 (t — 40.3) (l>) 



The following table shows how these two formulae represent the 

 observed Pleiades scale values. The numbers in the fourth column 

 are means from the two impressions on the plate. 



* Ann. N. Y. Acad. Sci., vol. vi, p. 271. 



