﻿Intelligence and Miscellaneous Articles. 



75 



that the black spot will gradually disappear, and ultimately vanish 

 in the gloom ; it will also be found that on different days, and differ- 



ent hours of the same day, the point at which the black spot vanishes 

 will vary with the intensity of the light. This point is read off on 

 the graduated scale, and thus we are enabled to measure the inten- 

 sity of the light at any required time. In taking an observation, it 

 would be well to state whether that portion of sky round the zenith 

 from which the cone of rays proceeds be clear or cloudy. 



It will be seen that the result obtained by this method is wot scien- 

 tifically correct, as it will be affected by the eyesight of the person 

 who makes the observation, but only in a slight degree. The me- 

 thod of measuring light, as just described, has been known to me for 

 upwards of three years. The hope that I should some day be en- 

 abled to make the instrument scientifically correct has hitherto pre- 

 vented me from making it public. As I understand that it is highly 

 desirable to have some means of estimating the changes in the light 

 which will occur during the total eclipse of the sun in August next, 

 I no longer feel justified in keeping in the background an instrument 

 which may possibly be of some slight assistance. 



XI. Intelligence and Miscellaneous Articles. 



ON THE DISPERSIVE POWER OF GASES AND VAPOURS. 

 BY M. CROULLEBOIS. 



r FHE measurement of the dispersive power of gases, which has long 

 ■"- been obscurely foreseen, has not yet been supplied by any phy- 

 sicist by the aid of a convenient and accurate experimental method. 

 The illustrious mathematician Cauchy, in the month of August 

 1836, even sent to the Academy a memoir which concluded, as a 

 necessary consequence of his able theory of light, that this dispersion 

 had no existence. Arago announced that in this there was an error 

 of fact, and he promised to publish a memoir on this subject con- 

 taining numerous delicate measurements ; but this memoir never 

 appeared, and it has not been met with in his papers. We are lost 

 in conjectures as to the method employed by this illustrious physicist 

 to measure the dispersion of colours in elastic fluids. Doubtless 



