PROCEEDINGS OF THE POLYTECHNIC ASSOCIATION. 639 



sight suspected the cause, and on examining the engineer on his sick bed 

 found that he could not distinguish all colors, but the engineer was not 

 aware until then of this deficiency. 



Prof. Vanderweyde said there were a few painters of quite a reputation 

 in this city who have some small degree of color-blindness, and mix false 

 colors in their landscapes. We notice the same defects in distinguishing 

 pitch as in color. Having had great practice in teaching music, he found 

 tliat about one in twelve could not distinguish the proper scale. When he 

 came to a certain note of the octave he always confounded it with another. 



Mr. T. D. Stetson said after the Norwalk accident he was employed to 

 investigate the cause. He took a locomotive and went over that part of 

 the route, and with regard to the signal he observed it was not high enough 

 to bring the sky as background, but the woods behind the signal tended to 

 deceive the eye. He had observed that there were different degrees in this 

 power of distinguishing color. He had once in his employ a person who 

 could not distinguish a green from a yellow line. He himself could not see 

 difierenccs in color as well as anotli^r gentleman in his office. He there- 

 fore concluded that the power to distinguish tints was not precisely the 

 same in any two persons, although there would be a very general agree- 

 ment as to a color. 



Residual Magnetism. 



Dr. A. Von Waltenhofen asserts that the amount of magnetism remaining 

 in the soft iron of an electro-magnet, after the cessation of the electric cur- 

 rent, is dependent on the manner in which the current is interrupted. The 

 amount is greatest after a gradual interception. The residual magnetism 

 is very soft iron is often of an opposite nature to that previously existing, 

 after the very sudden interruption of a strong current. This seems to him 

 to furnish strong proof that magnetism is not caused by the separation of 

 two fluids, but by the motion of magnetic molecules, to which is opposed a 

 certain amount of frictional resistance. He compares each magnetic mole- 

 cule to a spring which is bent back. If suddenly released it will return to 

 its original position, or go beyond it; but if gradually released it will not 

 go quite to its primitive place. 



The Chairman added — The power to retain magnetism in soft iron in- 

 creases in greater ratio than the mass. For this reason persons who had 

 made small working models of electrical machines were often disappointed 

 on finding that similar machines, njade on a large scale, did not exert 

 increased power in proportion to the increased size. 



The Chairman then presented the following interesting compendium of 

 recent scientific experiments and discoveries: 



Steel Boilers. 



Important experiments have been made in Prussia with steel boih^rs. Two 

 boilers, each 30 feet long and four feet in diameter, without flues, wore 

 placed side by side. One was made of steel plates, one-fourth of an inch 

 thick, the other of iron plates 0.414 of an inch in thickness. The steel 

 boiler was tested by hydraulic pressure up to 195 pounds per square inch. 

 Both boilers were worked for about a year and a half under 66 pounds pres- 



