18451 WRITINGS OF JOSEPH HENRY. 233 



path of the discharge. Indeed, he conceives that most of 

 the mechanical effects often witnessed in cases of buildings 

 struck by lightning, may be referred to the same cause. 

 In the case of a house struck within a few miles of Prince- 

 ton, the discharge entered the chimney, burst open the flue, 

 and passed along the cock-loft to the other end of the house; 

 and such was the explosive force in this confined space, that 

 nearly the whole roof was blown off. This effect was in all 

 probability due to the same cause which suddenly expands 

 the air in the experiment with Kinnersly's electrical air 

 thermometer. 



ON COLOR-BLINDNESS.* 



(From the Princeton Eeview, vol. xvii, pp. 483-489.) 



Juhj, 1845. 



It is an interesting fact in reference to the dependence of 

 at least one class of our knowledge — on sensation, that many 

 persons are born with defective vision and yet remain for 

 years of their lives without being conscious of the deficiency. 

 We know a gentleman who had probably been always near 

 sighted, but who did not discover the peculiarity of his 

 vision until the age of twenty-five, when it was accidentally 

 made known by looking at a distant object through a con- 

 cave lens. Many persons whose eyes are sound and capable 

 of exercising the most delicate functions, are permanently 

 unable to distinguish certain colors. And the number of 

 such persons is much more considerable than we would be 

 led to imagine from the little attention this defect of vision 

 has excited. It is often unknown to the individual himself, 

 and indeed only becomes revealed by comparing his powers 

 of discriminating different colors, with those of other persons. 



* 1. Observations on color-blindness, or insensibility to the impression of 

 certain colors. By Sir David Brewster. Philosophical Magazine. 



2. Memoir on Daltonism, (or color-blindness.) By M. Elie Wartmann, 

 Professor of Natural Philosophy in the Academy of Lausanne, &c. Scien- 

 tific Memoirs. 



