92 SECTION PHYSICS. 



simply to say that that assumption is made in order to enable another 

 hereafter to calculate what is the probable radiating power of the moon. 

 I placed a sheet of glass so as to intercept the rays of the moon, and 

 then I found the heat was all intercepted except ten or twenty per 

 cent, showing that the moon's heat was of a different quality and lower 

 refrangibility than the sun's heat, because the sun's heat on an average 

 gives eighty per cent, of the power through a glass against ten to twenty 

 per cent, of the moon's rays. I might also add that the concentration 

 obtained by this apparatus is very large. The moon's light spread on 

 a three feet diameter is concentrated on a third of an inch, which would 

 represent a concentration of the moon's light of about 1 1000 times, and 

 allowing for loss by reflection by the large mirror it would be about 

 4 $00 times. 



The CHAIRMAN : I believe Lord Rosse will also describe to us a 

 photometer, which is also on the table. 



ZOLLNER'S ASTRO-PHOTOMETER. 



LORD ROSSE : This is not a piece of apparatus with which I am 

 particularly connected, but I have brought it for description, because 

 it appears to be very little known in the British Isles. It is a German 

 instrument, devised by Professor Zollner. Our climate is not at all 

 well suited for photometric observations, because of the variability 

 of the transparency of the atmosphere, and the number ot clouds 

 which perpetually interfere with the observations. But this instru- 

 ment has been used in the German climate very generally, and 

 appears to rank very highly. Its primary object was to compare the 

 light of various stars. There is one considerable difficulty in com- 

 paring the light of stars, which does not occur in some other 

 photometrical experiments, viz., that you have to compare them as 

 accurately as possible when they are of different colours, and it is a 

 problem, therefore, to which only an approximate solution can be 

 given. In comparing two stars, you have simply to equalize the 

 colour as far as you can of the lamp which is used, and then, by 

 trying backwards and forwards, to seek to get the mean result as 

 accurately as you can. There is a small pin-hole which admits the 

 light of the lamp. It passes first through a double concave lens, then 



