ZOLLNERS ASTRO-PHO TOME TER 93 



through a Nicol prism, then through a quartz plate, by means of 

 which the colour is varied, and then through two more Nicol prisms. 

 The pin-hole forms a minute point of light, and there is also a double 

 concave lens which brings the light to a focus. The light from the 

 natural star comes through an object glass, and is seen by the 

 side of the artificial star, reflected by a diagonal clear plate of 

 glass, whilst that of the star passes through the glass with very little 

 diminished brilliancy. The size of the pin-hole may be varied, there 

 being six different sizes attached to the instrument. There are various 

 telescopes used with it; this is the smallest size, a very small one 

 being used for very brilliant objects. For observing Venus you require 

 a very small aperture, or else you never get an image faint enough to 

 compare. The concave lens is to make the light smaller and more 

 like the light of a star. Passing through the first Nicol, by turning it, 

 in combination with the quartz, you are able to vary the colour. It 

 then passes through the other prisms, which can be turned round, 

 and you can thus vary the intensity according to the well-known law. 

 It is usual to take four readings, and then find the average. Professor 

 Seidell, of Munich, who worked at the stars with this instrument, 

 obtained complete observations by four readings within the limits 

 of probable error of about 8f per cent. I may mention that the error 

 of one night's observation of the moon's heat by the thermopile was 

 about 10 per cent, on an average. It is rather less accurate than this 

 instrument was in Professor Seidell's hands, but I do not think, with 

 the practice I have had with this, I could get quite as good a result 

 as with the thermopile. Here is another piece of apparatus attached 

 to it, which is used for measuring the moon's light. It is similar to 

 that described above for forming the artificial star, except that there 

 is no quartz plate. In using it you see in the focus of the eye-piece 

 three stars, two from the lamp one coming from the back surface 

 of the lamp, and the other from the front, and one of the moon; 

 and a very practised eye would be able by changing the position of 

 the Nicol to make the image of the moon intermediate between the 

 brilliancy of the two, the difference in their light being due simply to 

 the absorption in passing twice through the thickness of the glass. 



The CHAIRMAN : Lord Rosse, who is a most worthy successor to 

 his illustrious father, has been good enough to explain to us some of 



