6i8 



NATURE 



[Oct 27, 1 88 1 



directions from tlie tun for some considerable distance all round 

 it, the totality of radiation from that portion of the heavens may 

 not be so much inferior to what it is when there is none of that 

 slight cirtus haze there. But still we have haze enough in the 

 lower regions, and besides that we have water in an invisible 

 state of vapour, and Dr. Tyndall has shown that that absorbs 

 with great avidity a portion of the heat rays. Those, ho^^ ever, 

 are mainly rays of very low refrangibility. Still the absorption 

 of these may very sensibly affect the totality of radiation received 

 from the tun. How then shall we, if possible, get rid of these 

 sources of disturbance? The best plan seems to be to take 

 observations at a considerable altitude, where if po-sible you may 

 get many thousand feet above the level of the tea and get rid of 

 the lower, dustier, hazier portion of the atmosphere, and get rid 

 also of by far the greater portion of the aqueous vapour, which 

 by itself alone would absorb a portion of the heat. That is what 

 the Committee on Solar Phy.-ics have attempted to do. We 

 contemplate having actinometric observations made in the 

 north of India. Of course, if observations are to be continued, 

 it is not sufficient to go to some high mountain. You mutt go 

 to some habitable place where the observer can live and be in 

 some sort of comfort. Now in the Himalayas you may get up 

 to many thousand feet and yet be still within reach of human 

 habitations ; or what is better still, if you cross tlie range and 

 go over into Thibet, you have there a high table land many 

 thousand feet above the level of the sea, with a sky usually 

 cloudless, and where observations of this kind may, it is hoped, 

 be made with success for a considerable period together, and 

 the result may, we hope, in time throw light on the question 

 whether or no there is in reality a change in the amount of 

 radiation received from the sun, and whether the amount of 

 that change is sufficient to make any material difference in the 

 meteorological conditions of our globe. 



I have spoken of meteorological elements in which various 

 observers suspected that they saw some indications at least of a 

 decennial, or nearly decennial, period. Speculations have been 

 made as to whether there is not a decennial period, or something 

 of the kind, traceable even in the occurrence of Indian famine^. 

 If so, there may be some very close relationship between the 

 solar spots ad these famines. At first sight one would be dis- 

 posed to say, "What possible connection can sun-spots and 

 famines have with one another ? You might as \\ell speak of the 

 connection between comets and wars ! " But when we go deeper 

 below the surface, and study carefully the phenomena presented 

 to our view, we see that a possible connection between such 

 apparently remote things as sun-spots and famines may not be 

 chimerical ; and there is no saying what practical application 

 may in the end result from a study of solar phenomena under- 

 taken in the first instance for a purely scientific object. 



A PRISMATIC OPTOMETER' 

 TT is well known that in the normal eye, with its accom- 

 modation relaxed, parallel rays of light, that is, those from 

 distant objects, are brought to a focus on the retina. Rays 

 from near objects are divergent, and if they enter such an eye 

 they are not brought to a focus on the retina, but would be at 

 some point behind it. In order that they may be to brought to 

 a focus and form a distinct image on the retina, an effort of 

 accommodation is necessary. This is perfomied by a small 

 muscle called the ciliary muscle, inside the eyeball, the ultimate 

 effect of whose contraction is an alteration in the shape and 

 perhaps the condition of the lens, which causes the rays to be 

 more strongly refracted, and brings them to a focus on the 

 retina. The effect is in fact the same as if a convex lens were 

 added to the optical system_of the eye. As age advances, the 

 muscle and lens become stiffer, and work with difficulty. They 

 are relieved of part of their work by putting a convex glass in 

 front of the eye. Hypermetropia is a condition in which the 

 axis of the eyeball is too short, compared w ith the refracting 

 power of the lens. In it an effort of accommodation is neces- 

 sary to see even distant objects clearly, and a still stronger effort 

 to see near objects. A person suffering from it requires convex 

 glasses. When both eyes are used together, the optic axes of 

 both are directed to the object, so that in looking at a distant 

 object they are directed parallel, and in looking at a near one 

 they converge. These movements are effected by the external 

 muscles of the eyeball, which are supplied by branches of the 



' "On a Prismatic Optometer." by Tempest Anderson, M.D., B.Sc, read 

 St the York meeting of the British Association. 



same nerve as the ciliary muscle. As a fact these movements of 

 the ciliary muscle .and of the external muscles of the eyeball are 

 associated, or habitually performed in conjunction ; that is, the 

 brain has become accustomed to send an impulse to the one set 

 of muscles proportionate to that sent by the other. Any dis- 

 turbance of this association can only be accomplished by a 

 distinct effort which, if severe or long contiimed, is apt to be 

 painful. Suppose a man has become presbyopic, i.e. his accom- 

 modation has gradually become stiff, and its range reduced. In 

 order to accommodate for rays from an object at the ordinary 

 reading distance of ten or twelve inches, he has now to 

 exert an effort equal perhaps to what he would have employed 

 when young on one four inches off, but the change has been 

 gradual, and the convergence of the eyes for twelve inches has 

 become associated with this amount of effort. If he now use 

 convex glasses of suitable power, the want of refracting power 

 is supplied, the effort of accommodation is reduced to its natural 

 amount, but the amount of convergence which has become asso- 

 ciated with this small effort is now insufficient, and the eyes, 

 instead of converging to twelve inches, converge on a poitit 

 several feet distant, so that double vision would be produced, 

 unless by a distinct effort the eyes M'ere converged more, and 



fye EndcfO^ TAnc/erson\ 

 Prismatic Optometer. 



main frame carried by F. graduated 

 prisms ; e, frame carrying ?', third prism 

 frames for lenses. 



:. rotating franie carrying i 

 :dge to separate 



this effort is often painful, and is expressed by the term that the 

 spectacles "draw" the eyes. After a time new asscciatioiis 

 are formed, and the spectacles can be used comfortably; but this 

 does not happen in all cases, and for these it is necessary to 

 grind the hnses on glasses of f rismatic section. The action of 

 the prism is so to bend the pencils of rays coming to the eyes 

 that they appear to diverge from a point corresponding to the 

 new focal distance of the eyes provided with the spectacles. 

 Sometimes the amount of prismatic effect required is calculated, 

 but the calcul'.tion, being based on general considerations, does 

 not always suit indiviolual persons ; at other times prismatic 

 glasses from a trial case, are combined with the calculated 

 spherical, or spherical and cylindrical glasses, until one is found 

 with which vision is comfortable. In many cases it is not neces- 

 sary to use glasses specially ground on prisms, but sufficient 

 to move the centre of the glasses nearer together. The glass 

 being thicker in the centre, looking through the part near the 

 edge produces an amount of prismatic effect which is often suf- 

 ficient. If concave glasses are U'ed, as in cases of short sight, 

 then they must be further apai t than the distance of the eyes, in 

 in order to produce this effect. The object of the instrument 

 exhibited is to find experimentally the amount of prismatic 

 power, and the distance of the centre of the lenses which is 



