PROCEEDINGS OF THE POLYTECHNIC ASSOCIATION. 917 



by John W. Draper, M. D., Professor of Chemistry in the Univer- 

 sity of New York," page 403 : 



" Before the paper of Mr. Towson, in the London and Edin- 

 burgh Journal of Science, had reached this country last November, 

 I had determined the proper focus for the daguerreotype. In 

 truth, every ray, except the yellow, leaves an impression on the 

 iodine. Theoretically, therefore, it would seem that, in order to 

 obtain perfect pictures, an achromatic lens is absolutely necessary. 

 A more attentive consideration of the matter soon convinced me 

 that lenses in which the chromatic abberration was uncorrected 

 might be employed, provided care was taken to remove the plate 

 from the camera at a certain period. For the impressions of light 

 upon the retina are solely regulated by intensity ; but in the action 

 of a decomposed beam on an iodized plate, time enters as an ele- 

 ment. Suppose, therefore, a plate be exposed in the camera, 

 during the space of five minutes, in light of a certain brilliancy, 

 if the focus has been adjusted to the focus for blue light, a neat 

 picture may be obtained ; for these being the rays in which the 

 action is at a maximum, they will have had time to make a com- 

 plete and perfect impression, whilst the red and violet rays will 

 not have had time to give any perceptible eftect. Upon these prin- 

 ciples I found that very sharp pictures might be obtained, not 

 merely by spectacle lenses an inch in diameter, but also by means 

 of lenses of three or four inches aperture, such as have since come 

 into common use. The first portrait I obtained last December 

 was with a common spectacle glass, only an inch in diameter, 

 arranged at the end of a cigar box. 



" The risk of failure by employing an uncorrected lens is- greater 

 than the risk by a good achromatic or a reflector." 



Flax Pulling Machine. 



Mr. Tyler exhibited an ingenious model of a flax pulling maehiney 

 which he stated will pull five acres of flax in a day, th,us enabling 

 one man and a horse to do the work of twenty-five men. The 

 machine costs less than one hundred dollars, and in Ohio, where 

 flax fibre worth millions of dollars is lost every year, because the 

 farmers cannot reap it economically, must be exceeediugly valuable. 



Prof. Vanderweyde occupied the rest of the evening with 

 remarks on Light and Sound, as the results of vibrations or undu- 

 lations, after which the association adjourned. 



