92 Mr. Pogson on the Madras Celestial Survey, 



Although a five foot telescope is to be regarded as the 

 standard size for the survey throughout, others will be freely 

 employed ; experience having long shown me the great ad- 

 vantage derivable from the use of different sized telescopes. 

 For the broadest zones, and finest moonless nights, a 2f inch 

 aperture, fitted on to the universal equatoreal stand before- 

 named will be employed ; while for moonlight nights, and 

 zones at low altitudes, the Lerebours' equatoreal will be used. 

 The eye will always be previously prepared by first turning 

 to the nearest variable star map in my " Atlas" at about the 

 same altitude, and carefully remarking the appearance of 

 the comparison stars of well known magnitude as given in 

 such map. Zones may thus be taken within very few de- 

 grees of the horizon, with ease and tolerable certainty ; and 

 presuming that the observer has the magnitude scale of the 

 five foot pretty well impressed upon his memory, the use of 

 another telescope simply demands the allowance for differ- 

 ence of penetration due to the various apertures, computed 

 by the formula: M — m— 5 (log. A — log. a) — in which M 

 and m stand for the apparent relative magnitudes of the 

 same star seen through the apertures A and a. Thus ; a star 

 of the 9 0 magnitude in the five foot, another of the 8*4 

 magnitude in the smaller telescope, and a third of 10 2 mag- 

 nitude in the Lerebours equatoreal, appear all equally bright, 

 under similar atmospheric conditions, and near the same al- 

 titude, when viewed with magnifying powers also propor- 

 tional to the aperture of each object glass. The powers 

 employed are from ten to twelve times the aperture of each 

 telescope. The reticles are all of one form : a thick vertical 

 bar, one edge of which bisects the field of view, leaving the 

 preceding half clear, for the better estimation of magnitudes. 

 Horizontal bars run across the following segment at inter- 

 vals of five minutes of arc apart ; the central and every 

 other bar being exactly half, and the intermediate ones one 



