300 



♦ KNOWLEDGE ♦ 



[April 10, 1885. 



jiupil haa always remained of one definite size at a prescribed 

 definite distance. 



The Diameter of the Pupil whjn- Stimilated by the 

 Light of the ivIoon. 



From numerous experiments on the iull moon, my own 

 pupil invariably measured the 17-OOth of an inch in 

 diameter. It becomes dilated to the same size when held 

 at 4 ft. G in. from a Sugg's stmdard candle. Now, the 

 square of 4 ft. G in. is equal to 20 2.5. Hence, according 

 to this showing, the light of the full moon is equal to 

 about the one-twentieth of that of the candle. Inotherwords, 

 it would require the light of twenty moons, to make that of 

 one candle. Dr. Wollaston made the light of the full 

 moon much weaker. According to his experiments, full 

 moonlight is equal tD only the y:fj^th of a caudle; but 

 some previous French experiments made it nearly three 

 times as much. 



On the occasion of the eclipse (Fig. 3) at 7.-10 p.m., 

 the pupil dilated to the 'IS iu., and at 8,-15 to the -19 in , 

 after which it was too dark to obtain another observation. 



Disparity in Size op the Normal Pupil ix Different 

 Persons. 

 It may be stated in limine that if from a light of the 

 same intensity the measure proves different in two persons, 

 that of both or of either may be equally correct. From 

 numerous experiments it results that, so far from the 

 pupil having the same size in ail, it is scarcely alike in two 

 consecutive cases, but that each person, it is more than 

 probable, has a pupil range of his own. This is shown 

 in the subjoined chart (Fig. 4), where the two measures 

 of the light's intensity, or of the diameters which we 

 assume to be so in two diflerent persons, are different, 

 yet both are equally reliable, for while the disparity con- 

 tinues throughout the day, the absolute numbers varying, 



i^-.ii C, 1SS4-. — Dull, Cloudy, no Sun, soms Sain. Eiashtees 

 OF Pupil cj nursDEEDias op an e;ch. 



e B =A.'8pnpiL 

 Sam of Diameter, = 113 ; Mean, = 9-4 



c— — c =B.'8pnpil. 

 Stun of Diameter, = 163 ; Mean, = 13o 



• San sadderJy. 



t San watery. 

 Fisr. 4. 



J Eaining hard. § 



their differences remain the same within the hundredth of 

 an inch or so, in all. 



It has been found useful to set the pupil in individual 

 cases to some ordinary standard, say a Sugg's candle, at 

 at one foot from the eye. My own pupil, when thus 

 tested, measured the 'l-j inch. We can now dispense with 

 the candle in miking a comparison with other lights, using 

 instead the diameter of our own pupil. In this way we 



become acquainted with the relative luminosity of certain 

 lights stated numerically, of which previously we had not 

 the remotest idea. The light of the full moon, for instance, 

 invariably dilates my pupil to the -17 inch, that of a 

 standard candle to the -15 inch, and that of a S.W. sky in 

 sunshine to the -07 By comparing these three lights, of 

 the moon, the candle, and the sky, by using the numbers 

 •17, -l-S, and -07, our ideas as to their relative intensities 

 become more definite. 



(To be continued.) 



RAIN. 



By Richard A. Proctor. 



(Continued from p. 255.) 



THE influence of the seasons remains yet to be mentioned 

 among the circumstances aflfecting the distribution of 

 rain over the earth's surface. The influence of the seasons 

 is diflerent in diflerent zones of the earth's surface. Under 

 the tropics the laws aflecting the fall of rain are much more 

 regular than elsewhere. On the ocean we have clear skies 

 when the trade-winds are blowing steadily, and heavy rain 

 falls by day over the intermediate zone of calms ; but on 

 the land we have regular dry and wet seasons within the 

 tropics. There is, properly speaking, no winter or summer ; 

 but applying these terms to the periods at which winter or 

 summer prevails in the temperate zones of either hemi- 

 sphere, we may say that the sky is serene in the winter, 

 becomes moist in spring, and the rainy season sets in when 

 the sun is near the zenith. Where there is a considerable 

 interval between the sun's passages of the zenith, as in 

 places not very far from the equator, there are two wet 

 seasons, both occurring in summer. In countries in which 

 monsoons prevail, however, the alternation of dry and wet 

 seasons depends on the winds. When the south-west mon- 

 soon is blowing over India, for instance, there is no rain on 

 the east coast, but abundant rain on the west coast. During 

 the north-east monsoon these conditions are reversed. A 

 little consideration will show that all the above-mentioned 

 seasonal variations within the tropics depend on general 

 laws already stated. 



Beyond the tropics there is le,-s regularity. The fall of 

 rain depends on the prevalence of certain winds which 

 bring moisture with them, and, these winds not blowing 

 with any regularity, the rainftill is similarly irregular. In 

 countries close to the tropics, there is a noteworthy dryness 

 in summer ; for this reason clearly, that iu summer the 

 trades blow over these regions, and bring with them "trade- 

 wind weather." Further north, however, though there 

 may be a tendency to the prevalence of north-easterly winds 

 in summer, this tendency is not so marked as to produce a 

 considerable defect of rain in the summer as compared with 

 the winter months.* 



In England we have" one cause aflecting the rainfall 

 which is worthy of special notice. I refer to the Gulf 

 Stream. The air above this warm stream is not only 

 warmer than the surrounding air, but is heavily laden with 

 moisture. When the western and south-western winds 

 loaded with the vapour of water begin to blow over England, 

 they precipitate their moisture in rain, as they encounter 

 the colder air, over the land ; but the manner in which this 

 happens is variably with the seasons, for in the winter 



* So far as my own observations extend, I should say that tho 

 two featnrea of our climate which may be most certainly depended 

 on — which, be it noted, is not saying much — are, heavy rains in 

 July, generally in the last fortnight, and serene weather during the 

 second week of November. 



