1888.] Presidents Address. xxxi 



Flora. As proposed by him the method required the co-operation of 

 observatories in the northern and southern hemispheres, and owing to 

 various unfavourable circumstances the observations in the southern 

 observatories were not sufficiently numerous and good. Another 

 attempt was made in 1882 on the planets Victoria and Sappho ; no 

 result has yet been deduced from these observations, but they will 

 certainly give a very fair result. A grand attack on the problem will 

 be made this year and next with the exquisite heliometer which has 

 recently been erected at the Cape, both independently by the method 

 of diurnal parallaxes and also in conjunction with similar instruments 

 in America and Germany, and results of the highest degree of 

 accuracy may be confidently expected. It is perfectly certain that 

 the minor planet method will give the most reliable determination at 

 the present time, but it is almost equally certain that the final value, 

 to be adopted many years hence, will be deduced from the perturba- 

 tion of che planets Venus and Mars under the action of the earth as 

 pointed out and already approximately done by Leverrier ; for this 

 method is one that ever increases in accuracy with the lapse of time. 



A very important series of experiments has been carried out by 

 Prof. Langley on the energy of the separate rays constituting the 

 lower part of the solar spectrum. In previous investigations the 

 spectrum had been formed by refraction through a prism, as no 

 instrument existed of sufficient delicacy to measure the heat of the 

 diffraction spectrum. But in the prismatic spectrum the rays are 

 much more crowded together at the red end than at the blue, and the 

 effect of previous investigations was consequently to indicate a 

 maximum of heat in the infra-red portion of the spectrum. Hence 

 arose the three curves of solar energy which we are accustomed to 

 find in the text-books, viz., the actinic, light and heat curves, with 

 maximum value at different parts of the spectrum. But in the 

 diffraction spectrum we have a normal arrangement of the rays side by 

 side without any crowding, and Prof. Langley by means of his bolo- 

 meter was able to measure the heating effect of each portion in detail. 

 He found that the point of maximum heat practically coincided with 

 that of maximum light, i.e., near the sodium lines in the yellow. We are 

 thus led to the conclusion that there is no difference of quality in the rays 

 themselves, but that the various effects produced by them depend on 

 the various capabilities which the bodies they impinge on possess of 

 turning to account the longer or shorter vibrations. 



From simultaneous observations at the summit and foot of Mt.. 



