TRANSACTIONS OF SECTION A. 465 



The proper lens being calculated, its spherical and cylindrical elements are 

 combined and put together before the eye. If it be the correct one, all the lines 

 are seen sharp at the same time. If not, further examination is made. 



The principal advantage of the first plan is that the adjustment, being made by 

 the motion of the wire screen, is continuous, and correcting lenses are not required 

 for measuring the refraction, but only for rendering the retinal image visible ; its 

 disadvantage that, as the rays are not parallel as they pass from the front lens, 

 past the mirror to the eye, it is necessary for the apparatus to be very near, and at 

 a determinate distance from the observed eye, otherwise the readings of the scale 

 are vitiated. This, however, is not a serious objection. 



In the second plan the rays in the corresponding position are parallel, and the 

 instrument can be held at any convenient distance, say 1 or 2 feet from the 

 observed eye, and the observer can get a view of the cornea at the same time as he 

 views the image, so that he can estimate the refraction at diiferent points of the 

 the cornea. 



It is hoped that this may eventually lead to the determination of the refraction 

 at different parts of conical cornea and other eyes with irregular astigmatism, 

 and the application of suitable lenses to them. 



Since writing the above, I find mention of an in.strument hy Coccius Stimmel, 

 with an optical arrangement on the same plan as my second, but I have not heard 

 of its being in use in this country. The makers are T. Cooke & Sons, York. 



6. On the Length of the Sun-spot Period.^ By Henkt Mdirhead, M.D. 



It is well known to all who take an interest in the phenomena of sun-spots that 

 men of science are far from being agreed as to the length of the period intervening 

 between one maximum epoch and another. I should think, however, that those 

 who wish to prove that our rain, our storms, terrestrial magnetism, harvests of 

 grain and vintage, with commercial crises, &c., depend a good deal on how the face 

 of the sun is covered, should first of all make sure of the real length of the sxm- 

 spot period. To help towards its fixation I beg to bring under the notice of the 

 Section the following table arranged from Wolf's latest corrected relative sun-spot 

 numbers, from 1770 till 1877, with accompanying tracing. The numbers are ar- 

 ranged to correspond with Jupiter's period of "11-863 years. Each vertical column 

 (save one) commences with the year in which Jupiter's heliocentric longitude is 

 263|°. The last column is the summation. The dates of the cycles were 

 chosen on the supposition that much of the sun's radiance arises from matter im- 

 pinging on his atmosphere — ^just as so-called star-showers light up ours — and that 

 when Jupiter is in or near the front of the sun's march in space {i.e. hel. long. 263|°) 

 he intercepts a good deal of the meteoric aggregations which would otherwise im- 

 pinge against the sun's envelopes, giving rise to sun-spots till sublimated. We 

 know that comets whose aphelia are about as distant as Jupiter's orbit have periods 

 of about five years ; so meteoric matter may be supposed to take about two and a 

 half years to come from Jupiter to the sun. Now please observe that the average 

 minimum sun-spot epoch occurs about two and a half years after Jupiter is at 

 R.A. 263|°, and the maximum about six years after the minimum, that is, when 

 we might expect Jupiter's intercepting influence to be least. But leaving out of 

 account this hypothesis, the fact remains — as the table and tracings^ show — that 

 the sun-spot period is not 10 years, nor 11, nor 11-1, but seemingly 11-863, con-e- 

 sponding with Jupiter's period. It is to this conclusion that I wish to call the 

 attention of meteorologists and others, who vnsh. to show a coi-respondence 

 between their various periodicities and those of sun-spots. In 1875 I brought 

 this before the Philosophical Society of Glasgow, and also produced evidence to 



' Will be published in extenso in the Proceedings of the PhilosopMoal Society of 

 Glaajow, vol. xii. 1880-81. 



* A diagram extending horizontally was also shown, 



1880. H H 



