146 ANNUAL OF SCIENTIFIC DISCOVERY. 



destroyed. The effect is complete when the water is smooth and the 

 angle with it 52, but is partial at other angles. The prisms are fitted 

 into common spectacle-frames, or may be used like an opera-glass. 

 2. Picture polariscope, the construction of which is the same as that 

 of the preceding, except that the admission of a perpendicular, instead 

 of a horizontal, ray is prevented. This instrument is to be used in 

 inspecting pictures hung in a bad light or too highly varnished. 3. 

 Polarizing diaphragm, which consists of two Nichols prisms, one being 

 fixed, and the other rotatory through an angle of 90 on the same axis 

 as the fixed prism. It is intended to be applied to the microscope, 

 and by it the light can be rendered more or less brilliant, as it is de- 

 sired. 4. Surgical polariscope to aid the oculist in examining the 

 cornea of the eye. It is a Nichols prism placed in a tube behind a 

 lens of long focus, which rotates freely on its own axis, to suit the 

 varying plane of the polarized ray from the cornea, so that all glare is 

 removed. 



At the New Haven meeting of the American Association , Professor 

 Snell, of Amherst, exhibited an instrument intended to illustrate the 

 vibrations of a molecule of common or unpolarized light. In common 

 light, the vibrations are not like those of sound, in the line of progress, 

 nor are they like sea-waves, perpendicularly across the line of prog- 

 ress, in a fixed direction, but they are across in all directions. The 

 instrument presented this changing direction of the vibration. A 

 small ball of ivory, in front of a black surface, is made to fly back and 

 forth with great rapidity, while the line of its motion gradually ad- 

 vances round the circle, somewhat like the hand of a clock. The 

 mechanism which produces this motion is merely a toothed wheel, 

 gearing into the interior of a toothed circle of about twice its diameter. 

 If the revolving wheel has just half as many teeth as the larger wheel, 

 the ball, as is well known, would describe a straight line, or ellipse, 

 in one fixed direction ; but if a single tooth be added to the wheel, the 

 line, or the axis of the ellipse, will slowly make progress round the 

 circle, and in this way the ball is made to produce the kind of oscilla- 

 tion proper to represent light. Professor Airy does not consider this 

 gradual progress of the axis of the ellipse as a precisely correct rep- 

 resentation of the case ; but the molecule should be conceived to vi- 

 brate many hundreds of times in one and the same position, and then 

 hundreds more in a new position, and so on. It is only necessary, 

 then, to imagine the ivory ball to vibrate with such rapidity as to 

 make a few millions of oscillations in each of its successive positions ; 

 or, to state a definite case, let it vibrate five hundred thousand times in 

 each angular second of its progress, and yet let the ellipse of vibration 

 make its entire rotation in the one thousandth part of a second of time, 

 still the whole will not be too great for the truth, since the number is 

 six hundred millions of millions per second. 



Professor Snell also exhibited an arrangement by which the lecturer 

 can enable his audience all at once to try the experiment of comple- 

 mentary colors in vision. The apparatus consists of two disks, one a 

 foot or more in diameter, and perforated by three openings, and the other 

 behind it painted with alternate whito and colored sectors. While the 



