1890.] 



On the Heed of the Moon and Stars. 



487 



independent adjustment, so that ultimately the focus is in the vertical 

 axis of rotation, and at the same time the cone of rays from the large 

 mirror is just not sufficient to cover the surface of the flat. 



Though there is a finder, it would hardly be safe to trust to this to 

 know when a star had just come on to the sensitive surface, and so I 

 have arranged a 1-inch total reflecting prism behind the metal block 

 under the magnet, which can be turned round so as to view the sensi- 

 tive surface, and any image in the small space round it, from either 

 side of the box. There is an oblong hole in each side for this 

 purpose, through which a low power eyepiece, carried by a bracket on 

 the metal block, projects ; the space round the eyepiece is covered in 

 by a separate shield, to prevent hot or cold air from entering the box. 

 The dark radiations from the pupil of the eye are entirely prevented, 

 by the three glasses, from reaching the sensitive surface, so that it is 

 possible to watch the image of any heavenly body quietly transit 

 across the disc or sensitive surface without disturbing the indications 

 by the heat of the eye. I have arranged a temporary small telescope 

 with a diagonal eyepiece, immediately above that of the chief tele- 

 scope. The small telescope shows that part of the scale to which 

 the spot of light is brought, magnified, so that without moving the 

 body or any part of the apparatus it is possible to watch a star come 

 on to the disc, and to see the effect on the scale, and thus to avoid 

 every source of error, at once. If in any case a star is observed to 

 transit over the disc time after time, and the index is not moved 

 through one-quarter of a millimetre (and I find on a perfectly clear 

 and quiet night there can be no doubt whether this is so or not, — I 

 should even have little doubt of a tenth of a millimetre), then it is 

 certain that the heat received was not sufficient to produce such a 

 deflection. .An equatorial star takes about 20 seconds to cross the 

 disc, while practically the whole deflection due to any source of heat 

 is produced in 5 seconds, and so, no matter how long the star might 

 be kept on there would be no gain, while, on the other hand, the longer 

 that it is necessary to leave the star on before practically the whole 

 deflection is produced, the greater is the uncertainty of the zero of 

 the instrument. The advantage of the short time constant, if I may 

 use this expression, is fully proportional to its smallness, if it is not 

 proportional to some higher pow r er of its smallness. 



I determined not to put up the apparatus in the doubtful atmo- 

 sphere of London, and I am fortunate in having been able to fix it in 

 my father's garden at Wing, in Rutland. The position is certainly 

 good, the altitude is about 400 feet, the climate is as dry as in any 

 part of England. The subsoil is oolitic limestone, containing a large 

 quantity of iron, and very firm (the foundations of many of the old 

 walls in the village are from 1 to 2 feet above the present level of the 

 ground, and are perfectly secure). There is not a house or building 



