136 MALUS. 



which our friend alleges, the instance of the light which 

 is engendered in a vacuum, by the aid of the voltaic 

 current, passed through simple substances, such as car- 

 bon, platinum, &c. 



.In the second part of the memoir Malus seeks to 

 establish that the different natures of various lights only 

 differ from each other in the greater or less proportion 

 of caloric which they contain. The red light would 

 thus be the most heating, the violet the least so, which 

 agrees with experiment. According to a singular opinion 

 professed by the author, all rays, if possessing a certain 

 high intensity, ought to produce the sensation of white- 

 ness.* 



The third part of the work is devoted to mechanical 

 consequences which result by analysis from the supposi- 

 tions explained in the first two sections. It may suffice 

 to say, that the author finds, like all the partisans of the 

 system of emission, that the velocity of light ought to be 

 greater in water than in air : every one therefore will see 

 how superfluous it would be now to go into a discussion 

 of the details of such a subject. 



* The " singular" opinion here ascribed to Malus is perhaps not 

 altogether without foundation, at least in some cases. It is certain 

 that while the prismatic spectrum of the white light of the clouds 

 present a clear yellow and green portion, that same portion, when the 

 direct rays of the sun are substituted, appears to the eye intensely 

 brilliant and white. And it is far from certain that in some other ex- 

 periments, which have been the occasion of some little controversy 

 and where the colour of certain parts of the spectrum has appeared 

 to undergo a change, the intensity of the light reaching the eye may 

 not be concerned. In fact, the sensation of colour is one so entirely de- 

 pendent on unknown physiological causes, that we can hardly venture 

 to predict what the result may be on different individual eyes, though 

 all the optical conditions may be precisely the same. It may not be 

 altogether \vithout a bearing on this subject, to remark the extremely 

 contradictory statements made by different observers as to the colour 

 of intensely brilliant meteors. Tr tin tor. 



