Proceedings of tee Polytechnic Association. 1071 



very scarce and dear. That is tlie test. Washington market is the 

 best barometer. 



Mr. Phin. — Perhaps so, but -I think yon will find the exceptions 

 pretty general too. Moreover, how would it be if the farmers throngh- 

 out the country were to take to the general production of some honey 

 producing plant, such as buckwheat ? 



Mesteeals. 



Dr. Parmelee read a letter from Prof. Julus G. Pohle, in which 

 was described a small collection of minerals presented by Prof P. to 

 the Institute. The first of these was a specimen of massive rutile, 

 discovered by one of our soldiers in the valley of the Shenandoah. 

 On analysis, it was pure oxyd of titanium. Another was a specimen 

 of native sulphide of antimony from Xew Brunswick, where a vein 

 nearly three feet in width exists. It contains a small j^roportion of 

 arsenic. A specimen of rock salt from the Island of St. Domingo 

 was also exhibited. In this island it exists in immense quantities. 

 The specimen exhibited contained ninet^Miine per cent of pure salt. 



The chairman, after some preliminary business, announced as the 

 regular subject of the evening, 



Light and Color, 



Dr. P. Yanderweyde took the floor and first called the attention 

 of the society to achromatism. Its derivation is from the Greek a 

 (privitive), and cht'o^na, color. Achromatism refers to those princi- 

 ples by which we are enabled to free instruments requiring lenses 

 from defects relating to color. These defects are, that objects viewed 

 through them appear colored. If stars or other heavenly bodies be 

 observed by the aid of a telescope, they will appear colored, and this 

 coloration is due to principles which were afterward explained by 

 the lecturer. The same defects are observable in the microscope. 

 The reasons for this are as follows : When rays of light fall upon the 

 surfaces of denser media than that through which they have been 

 traveling they are refracted, and this refraction is not always the 

 same. It varies with the direction in which the ray strikes the sur- 

 face. Descartes has investigated the subject, and determined tlie 

 law of this variation. But rays of light are not only refracted, but 

 dispersed into various colors. This was treated of at a former meet- 

 ing, and does not need to be here enlarged upon. It was found, 

 however, within the past few years, that the dispersive power of 



