ON THE INTENSITY OF SOLAK BADIATION. 159 



With a view to increasing the eflfect of the radiation from the sun on 

 the thermometer d its bulb was ordered to be of green glass of a particu- 

 lar kind. This, as we ascertained from Mr. Casella, who made the instru- 

 ment, occasioned a great deal of trouble, as not only had a pot of green 

 glass to be made specially for the purpose, but many thermometers broke 

 in the process of construction, the fracture taking place at the junction of 

 the green glass of the bulb with the colourless glass of the stem. In a 

 future instrument we should not think of encountering these difficulties, 

 since, as will presently appear, our researches led us to the conclusion 

 that little advantage, if any, was gained by the substitution of green for 

 colourless glass in the construction of the bulb. 



The principle of the instrument was to make any point of the radia- 

 tion thermometer (d) look, so to speak, in all directions outwards at an 

 envelope of uniform temperature, except as to directions lying within a 

 very small solid angle (that subtended by the hole), within which the 

 sun's rays were admitted. If the direct rays of the sun had been used 

 the solid angle in question could only be made small on condition of 

 admitting only a very small amount of the sun's rays, which would not 

 have sufficed to raise the temperature of d sufficiently above that of A, b, c. 

 To reconcile the two conditions of allowing the bulb of D to be almost 

 wholly surrounded by the copper envelope, and at the same time permit- 

 ting a sufficient amount of solar radiation to fall upon it, a lens was intro- 

 duced, mounted on a stem perpendicular to the front face, to which face 

 the plane of the lens was parallel, and from which the lens was distant by 

 its focal length. In this way the necessary hole in the envelope need 

 hardly be wider than the image of the sun, though it was convenient to 

 allow some margin in order to provide for the contingency of the pointing 

 of the instrument not being very exact. The lens was provided with 

 two diaphragms for optional use, one having twice the area of the other. 



The observations which have been taken at intervals during the past 

 year with a view to test the practical working of Stewart's second actino- 

 meter have been made by Professor McLeod ; the reduction of the obser- 

 vations has been mostly done by the Chairman, with whom also Professor 

 McLeod has been in frequent communication as to the lines of inquiry. 

 In consequence of other engagements, the observations have not yet been 

 subjected to so complete a reduction as the care with which they have 

 been made deserves ; but enough has been done to serve as a guide to the 

 inquiry, and to permit of some general conclusions as to the^behaviour of 

 the instrument. 



As has been already stated, the instrument was intended for use as a 

 statical actinometer with permanent exposure. But it seemed desirable 

 in the first instance to study the march of the thermometers when the 

 instrument was first exposed to radiation from the sun, or the sun's rays 

 were cut off after it had been exposed for some time. This seemed to 

 hold out a better prospect of obtaining a thorough insight into the work- 

 ing of the instrument than if it had been at once used as a statical 

 actinometer ; besides which the latter use would have involved some 

 outlay in the way of providing some sort of equatorial mounting and 

 clock movement, and it did not seem desirable to go to the expense of 

 this unless preliminary testinj; showed that the instrument was likely to 

 be successful when used as a statical actinometer. 



The temperature of the case was determined from the readino-s of the 

 thermometers a, b, c by taking first the mean of a and B, and "then the 



