C8 Natural- Philosophical Collections. 



When the pipes were eirlpty, which happened to be the case several times dur- 

 ing the course of the observations, and when in consequence their outer and inner 

 surfaces were in contact with the air, the author of the memoir remarked that the 

 elongation in metres and in degree of temperature was about a tenth less than 

 the elongation of cast-iron, such as it was observed in 1785, in England, by 

 Major-Gencral Roy. This evidently arises either from the difference of expansi- 

 bility of the two kinds of cast-iron on which the expeiiments were made, or from 

 causes which opposed themselves to the free motion of the tubes upon the surface 

 of their supports. In giving a detailed account of the various circumstances of 

 his observations, the author also pointed out the precautions necessary to be tak^n 

 for laying in a solid manner, in the streets of a town, the pipes destined for diiS- 

 tributing water to its different parts, and concluded his memoir with proving,' 

 by facts observed some years ago at Paris, how much more advantageous it is to 

 lay these pipes in arched galleries, than to place them in the ground under the 

 pavement of the streets. 



On some Properties of the Impressions prodticed by Light upon the Organ of 

 yision The following results have been obtained by M. J. Plateau from a mul- 

 titude of experiments, remarkable for their precision. 



Section First : 



1. Every sensation requires an appreciable time for its complete formation, as 

 well as for its entire disappearance. 



, 2. The sensations are not effaced suddenly, but diminish gradually in intensi- 

 ty. 



3. When a sensation is disappearing, the progress of its decrease is the less 

 rapid as it approaches nearer to its termination. 



4. The different colours, observable by the simple light of the day, produce 

 sensations which differ very little in their whole duration. In this respect, the 

 colours ought to be arranged in the following order, commencing with that which 

 produces the most durable sensation : white, yellotc, red, blue. 



5. The total duration, noted from the instant when the sensation has acquired 

 its full force, to that when it is scarcely sensible, is, at least, 0",34 mean term. 



G. Lastly, it results accidentally from his experiments, that the principal co- 

 lours ought to be ranged, with relation to the intensity of their sensations, in the 

 following order : white, yellow, red, blue, commencing with that which produces 

 the most energetic impression. 



Section Second : 



1. Additional facts confirm the last of the preceding results. 



2. The visual angle under which his eye ceased to perceive the different co» 

 lours, were as follows : 



In the shade. In the sun. 



White, - - 18" 12" 



, YeUow, - - 19" 13" 



Red, - - 31" 23" 



Blue, - - 42" 2G" 



The angles observed in the sun being nearly two-thirds of the -corresponding 



angles in the shade. 



3. When the sensations of two different colours succeed each other alternately, 

 on their return, with a rapidity insufficient for the formation of a simple sensation, 

 vivid gradations of colour are generally formed, differing from the two employed, 

 as well as from their joint colour ; we may even, by this means, produce a beau- 

 tiful white from the employment of yellow and blue. 



4. When two sensations alternate with each other, with such rapidity that they 

 seem to form but one, this latter is not always the same as the material mixture 

 of the two colours employed : thus, in combining in this manner, and in certain 

 proportions, the impression of yellow with that of deep blue, we produce a perfect 

 gray, without the least shade of green. 



5. The sensations of certain colours (perhaps with the exception of yellow) 



