35 



This enabled me to shut off any proportional part of the 

 aperture of the telescope from one half to the whole. 



In order to equalise the conditions, so far as to have 

 equal visual areas in comparison, I covered the field-bar 

 of the eye-pieces used with tinfoil (blackened) leaving 

 open (for finding the object) a segment only, about one- 

 third of the field. In the centre of the tinfoil I pierced 

 a small hole with the point of a fine needle. 



In observing, I used a Barlow lens in conjunction with 

 the eye-pieces, so as to enlarge the focal image. I at first 

 employed the sun-prism (the image being viewed by 

 reflection from the first surface of clear glass), thereby 

 getting rid of the greater part of the illumination. This 

 was still further reduced by a 6-inch stop to the telescope. 

 For comparison, a lamp was enclosed in a cupboard having 

 an aperture covered with paper. The image of Mars 

 being brought to the needle bole, the shutter was gradu- 

 ally closed until the brightness of the image equalled that of 

 the lamp-light. The different readings gave an average of 

 90dcg. Dealing with Jupiter in the same way, the average 

 was 29deg. 



I next varied the experiment by removing the sun-prism, 

 and substituting a dark wedge (a make-shift affair for the 

 occasion, composed of two slips of glass, in contact at one 

 end, the other ends being slightly separated, the intervening 

 space being filled with Canada-balsam mixed with lamp- 

 black). A suitable darkening with this being obtained, it 

 was retained in position, and the shutter used for equalising 

 the illumination. 



In this experiment each image was reduced to the minimum 

 of visibility. The averages of the readings were 102deg. for 

 Jupiter, and 146deg. for Mars. 



Between these two sets of measures there is considerable 

 discordance ; but they both agree in making Jupiter's 

 surface much the brighter of the two ; in the first set as 90 

 to 29, or nearly three times ; and in the second as 146 to 102, 

 or nearly 1|. Taking the mean we have as 118 to 65, or 

 nearly double. 



Their relative distances from the sun at that date were as 

 15,747 to 54,561. The proportion of the squares of these 

 numbers is almost exactly as 1 to 12 ; Jupiter, then, instead 

 of being -^ the brightness of Mars, is by the above measure 

 nearly double, or about 22 times as bright as he ought to be 

 by the laws of radiation. 



The near conjunction of Jupiter and the Moon, on 7th 

 July, furnished opportunity for comparison between these 

 bodies. On this occasion the readings averaged 180deg. for 



