lo THE PRESIDENTIAL ADDRESS 



method of spectroscopy, into which I do not enter further. But 

 there is an interesting class of cases where we cannot afford to 

 sacrifice the form of the object entirely to colour discrimination. 

 Consider, for example, the prominences of the sun's limb, which are 

 so well seen against the darkened sky of an eclipse, but are altogether 

 lost in the glare of the sky at other times. In order to see them 

 prismatic dispersion is made use of, and separates the mono- 

 chromatic red light of hydrogen from the sky background. A slit 

 must be used to cut off the latter : but if it is too narrow the outlines 

 of the prominence cannot be seen. By using a compromise width 

 it is possible to reconcile the competing requirements in this com- 

 paratively easy case. Indeed, M. B. Lyot, working in the clear air 

 of the observatory of the Pic du Midi, where there is less false light 

 to deal with, has even been able to observe the prominences through 

 a suitable red filter, which enables the whole circumference of the 

 sun to be examined at once, without the limitations introduced by a 

 slit. A much more difficult problem is to look for bright hydrogen 

 eruptions projected on the sun's disc, and at first sight this might 

 well seem hopeless. A complete view of them was first obtained by 

 photography, but I shall limit myself to some notice of the visual 

 instrument perfected by Hale and called by him the spectrohelio- 

 scope. A very narrow slit has to be used, and hence only a very 

 small breadth of the sun's surface can be seen at any one instant. 

 But the difficulty is turned by very rapidly exposing to view successive 

 strips of the sun's surface side by side. The images then blend, 

 owing to persistence of vision, and a reasonably broad region is in- 

 cluded in what is practically a single view. I must pass over the 

 details of mechanism by which this is carried out. 



There are now a number of spectrohelioscopes over different 

 parts of the world, and a continuous watch is kept for bright erup- 

 tions of the red hydrogen lines. Already these are found to be 

 simultaneous with the ' fading ' of short radio waves over the 

 illuminated hemisphere of the earth, and the brightest eruptions 

 are simultaneous with disturbances of terrestrial magnetism. At 

 the Mount Wilson Observatory such eruptions have been seen at the 

 same time at widely separated points on the sun, indicating a deep- 

 seated cause. There are therefore very interesting and fundamental 

 questions within the realm of this method of investigation. 



We have so far been mainly considering how we may adapt our 

 vision for objects too small or too far off for unassisted sight, and for 

 colour differences not ordinarily perceptible. This is chiefly done- 

 by supplementing the lens system of the eye by additional lenses 

 or by prisms. We cannot supplement the retina, but in certain 

 cases we can do better. We can substitute an artificial sensitive 

 surface which may be either photographic or photoelectric. 



