442 TRANSACTIONS OF THE AMERICAN INSTITUTE. 



same as nitric acid will have upon copper. Now what nitric acid will do 

 in a few hours, oleic acid will do in a few years; so if time enough is given, 

 we can get the same effect from both. 



The subject for the evening, " Novelties at the Fair," was then taken up. 



The Globe Lens for the Camera. 



The Chairman. — A very important advance in the science of optics haa 

 recently been effected, for which we are indebted to Messrs. Harrison and 

 Schnitaer, of this city, who have constructed lenses which can copy straight 

 lines. It was supposed that this could not be done, but we have the fact be- 

 fore us. Maps are correctly copied by this instrument. It is used in the 

 office of the Coast Survey. These lenses are on exhibition at the fair, and 

 also photographic pictures taken by them. They have been sent to Eu- 

 rope, and we have the satisfaction of hearing the artists there admit that 

 they produce the much desired effect of an exact copy without perceptible 

 distortion. 



Prof Vanderweyde. — Many here no doubt recollect when the daguerreo- 

 type was first discovered; the large lenses that were used in the camera, 

 and that to make any kind of a sharp picture, diaphragms of very small 

 apertures had to be employed. These lenses were principally plano-convex 

 of some three or four inches in diameter. After these came the achroma- 

 tic lens, made of flint and crown glass. In this style two achromatic lenses 

 were used, the front one being cemented together with balsam of fir; and 

 in the back lens the flint was separated from the crown glass by a 

 ring about an eighth of an inch in width. These lenses have been in use 

 for the last fifteen years. But the great problem to be solved was to do 

 away with spherical aberration. Some yean; ago Prof Petzval, of Vienna, 

 made a series of calculations with a view to eliminate this defect in lenses; 

 and singular as it may appear, the theory which he deduced from these 

 calculations is the same which Mr. Harrison has now practically carried 

 out. In studying this subject, Mr. Harrison took the hint which nature 

 gives of a perfect lens, the human eye, which, as a piece of mechanism, is 

 unequalled for its beauty and perfection, and forms a perfect camera. The 

 similitude between its principal parts and the contrivances employed for 

 taking pictures is very striking. The human eye is a chamber colored 

 black on all sides except the back part known as the retina, which is a thin 

 white membrane, and receives tiie image from the lens in front. The pupil 

 acts like the diaphgram, collects the scattered rays, and brings them where 

 they unite in forming a bright and distinct picture of external objects. Mr. 

 Harrison's lenses is nothing more than an artificial imitation of the human 

 eye. Samples of these lenses are on exhibition at the fair; they are about 

 2|-inch focus. The ordinary lenses, in order to take a picture of three inches 

 diameter, usually have to be of six inches focus. 



The following article, by Mr. Colman Sellers, of Philadelphia, fully ex- 

 plains the advantages gained by this lens : 



The Glube Lens for photographic cameras, patented by Messrs. Harrison 

 .and Schnit/er of New YorU, is now attracting so much attention and is the 

 Bubject of such contradictory statements, that a brief notice of it by one 



