300 PEOCEEDINGS OP THE AMERICAN ACADEMY 



VII. 



CONTRIBUTIONS FROM THE PHYSICAL LABORATORY OF THE 

 MASSACHUSETTS INSTITUTE OF TECHNOLOGY. 



No. I. 

 FOCI OF LENSES PLACED OBLIQUELY. 



By Prof. E. C. Pickering and Dr. Chas. H. Williams. 



Presented, Feb. 9, 1875. 



The following experiments were suggested by noticing that the spot 

 of light of a reflecting galvanometer was thrown out of focus when 

 the mirror turned slightly. The image in this case was formed by a 

 lens near the mirror ; and, to obtain a distinct image, it was found that 

 the ends of the scale must be brought much nearer the mirror, owing 

 to the obliquity of the rays ujwn the lens. It was further noticed that 

 the focus was greatly altered when the slit used was placed horizon^ 

 tally instead of vertically. To study the matter more carefully, the 

 following apparatus was constructed. 



To one end of a board about five feet long was fitted a small telescope; 

 ten inches from this was placed a graduated circle, which rested hori- 

 zontally on the board ; and at its axis was fixed a screen of sheet iron, 

 which stood vertically and had a hole in the centre, on a level with the 

 telescope. 



On one side of the screen, and covering the hole, was fixed a bi- 

 convex lens of 25.5 inches focus, so placed that, when the graduated 

 circle \a as moved, the lens turned about a vertical axis which coincided 

 with that of the circle. At the farther end of the board was placed 

 a small gas flame, and between the lens and gas was a screen which 

 moved over a scale giving the distance in inches fi-om the axis of the 

 lens. At the centre of this screen, on a level with the lens, telescope, 

 and gas flame, and in the same straight line, two fine slits wtire cut, 

 one vertical the other horizontal, intersecting each other in the middle ; 

 and in these slits filaments of silk were stretched lengthwise, to aid in 

 focussing. 



To use the instrument, the gas was lighted, then the screen with the 

 cross-hairs was placed at the principal focus of the lens : that is, 25.5 



