April 17, 1885.] 



♦ KNOWLEDGE • 



323 



one. All its movements liave been found to harmonise 

 with our anticipations. Its fluctuations are synonymous 

 with the light's tluctuations, and it is possilile to control the 

 size of the one by selecting the quality of the other. In 

 short, had not tlie pupil been submitted to these periodical 

 tests it might still have remained to us a mere passive 

 organ, incessantly on the shift in obedience to the light 

 which controls it, but altogether useless as a measure of 

 that light. 



Absolute Intensities. 



Hitherto we have restricted our observations to lights of 

 relative intensities, by which we have been enabled to 

 compare the intensity of two or more sources of light with- 

 out knowing the absolute illumination of either of them. 

 It remains to inquire whether, in response to a light of 

 absolute brilliancy, the pupil is enabled to indicate in 

 definite magnitudes the quality of the light, varying as it 

 does with the square of the distance from the luminous 

 body in obedience to the general law of radiant forces. 

 Whether, for instance, at, say, twice the distance from any 

 luminary, the pupil will so dilate to a given magnitude 

 which shall exactly represent in a given fraction the weaker 

 light known to be of one-fourth the intensity. 



The most correct method of investigating the photometric 

 powers of the pupil with which I am acquainted is obtained 

 by the aid of a candle-flame, the absolute intensity of 

 which, whatever that may be, is always the same ; which 

 may be used in units, or multiples of those units (area), 

 and which may be removed at pleasure to any distance 

 from the eye. These properties are combined in a Sugg's 

 standard candle, which, consuming as it does two grains of 

 its own material by weight per minute, emits an agreeable 

 light. 



To obtain the proper effect we place a white surface 

 perpendicularly behind the candle one foot from the eye. 

 We can now easily find the two discs seen through the 

 photometer by looking through the flame upon the white 

 screen, both eyes being open. Measured in this way my 

 own pupil becomes dilated to the 15 of an inch, and 

 never varies. On now removing the candle to a distance 

 of two feet the eye will receive a light equal to a fourth of 

 the intensity (2-=4). Hence if a single candle illuminates 

 a body to a certain extent at the distance of a foot, it 

 would require four candles at the distance of two feet to 

 produce fqual illumination. Accordingly, on placing four 

 candles at two feet my pupil remains steadily dilated to the 

 •15 inch as before. 



This experiment affords strong corroborative evidence of 

 the appreciative powers of the pupil in e.xactly measuring 

 the intensity of four lights, which, at a certain definite 

 distance specified by a well-known law, should be exactly 

 equivalent to the intensity of one light only at another 

 distance according to the same law. 



The absolute brilliancy (B) of a luminous body is pro- 

 portionate to the absolute intensity (I) of the luminary, 

 multiplied by the luminous points of its illuminating 

 surface {e.g., number of candles) (S), divided by the square 

 of the distance (D-) of the illuminated object from it. 

 The brilliancy of the illumination will then be e,xpressed 

 by:- 



I X S 

 D- 



B = 



When by the formula the absolute brilliancy of a candle 

 at the distance of one foot from the eye will be equal to : — 



•15 X 1 



B = ^ = 15 in pupil measure. 



12 ^ *^ 



And of four candles at two feet from the eye will be 

 equal to : — 



B = -iii^ = -15 



2- 



Where the equality in the two brilliancies obtained 

 from different sources at dilTerent distances shows the 

 remarkably appreciative ])0wer of the pupil. 



It will be observed that the process of equalising the two 

 illuminations becomes at the same time the mcasun^ of the 

 number of candles used in the experiment. Hence, by an 

 extension of the same principle, the candle-power of any 

 luminous body may be found ; thus, if one candle at one 

 foot is e(iual in intensity to four candles at two feet, it will 

 be similarly equal to nine candles at three feet where the 

 number of candles in each case is found by taking the 

 square of the distance. Let it be required, for instance, to 

 find the candle-power of a gas-flame. Suppose, now, that 

 the distance required to equalise the illuminatiou.s (that is 

 to make both appear equal to the size of the pupil at one 

 foot from a candle) is equal to six feet, then the square of 

 six being equal to thirty six, the brilliancy of the gas flame 

 will be equal to that of thirty-six candles. 



OTHER WORLDS THAN OURS. 



A WEEK'S CONTEESATION ON THE PLURALITY OF 



WOBLDS. 



By Mons. de Fontenelle. 



with notes by richard a. proctor. 



THE FIFTH EVENING. 



(Continued from p. 283.) 



" AT'OU have no reason," said I, " to pity the inhabitants 



JL of a comet ; yet I sujipose you will think their con- 

 dition lamentable, who inhabit a vortex, whose sun comes 

 in time to be quite extinguished, and consequently live in 

 eternal night." 



" How ! " cried the Marchioness, " can suns be extin- 

 guished 1 " 



" Yes, without doubt," said I; "for people some thousand 

 years ago saw fixed stars in the sky which are now no more 

 to be seen ; these were suns which have lost their light, and 

 certainly there must be a strange desolation in their 

 vortexes, and a general mortality over all the planets : for 

 what can people do without a sun? " 



" This is a dismal fancy," said the lady. " I would not, if 

 I could help it, let it come into my head." 



" I will tell you, if you please," I reply 'd, " what in this 

 particular is the opinion of learned astronomers. They 

 think that the fixed stars which have disappear'd are not 

 quite extinguish'd, but that they are half suns— that ia, 

 they have one half dark, and the other half light ; and 

 turning round upon their own axis or center, they some- 

 times show us their light side, and afterwards turning to 

 us their dark side, we see them no more. To oblige you, 

 madam, I will be of this opinion, because it is not so harsh 

 as the other ; but I cannot make it good, but in relation to 

 some certain stars; because, as Huygens has lately observ'd, 

 those stars have their regulated times of appearing and dis- 

 appearing, otherwise there could be no such thing as half 

 suns. But what shall we say of star.^ which totally disap- 

 pear, and never show themselves again after they have 

 finish'd their course of turning round upon their own axis'! 

 You are too just, madam, to oVjlige me to believe, that such 

 stars are half suns. However, 1 will try once more what I 

 can do in favour of your opinion : The suns are not extinct, 



