Mr. J. C. Douglas on an Optical Experiment. 43 



From the first example we see that by using 64° prisms in- 

 stead of 45°, we obtain more than double the dispersion, with 

 even less loss of light, while in the second case seven 64° prisms 

 prove much more efficient than twelve of 45°. 



All these calculations seem to point to the superiority of 60° 

 prisms over those of 45°. A much greater angle is objection- 

 able from the increased distortion produced by the slightest im- 

 perfection in the refracting faces. 



In prisms admitting the same amount of light, the more acute 

 the angle the less is the quantity of glass and the less the area 

 of each face. The ground might be taken that a 45° prism could 

 be made larger than one of 60° at the same expense, and thus 

 the difference in light remedied. In this case, however, it would 

 be necessary to enlarge the telescope, number of prisms, and in 

 fact the whole instrument. Even supposing this change made, 

 the prisms of larger angle preserve their superiority, though not 

 in so marked a degree. The calculation is readily made by mul- 

 tiplying the transmitted light by cos i, as is done in the above 

 examples. 



The index of refraction varying with the refrangibility of the 

 rays, the dispersion, loss of light, &c. would vary in different 

 parts of the spectrum. The change would, however, be small, 

 and could be determined, if necessary, by merely altering n. 



Institute of Technology, Boston, 

 February 29, 1868. 



IX. An Optical Experiment. By John C. Douglas, East-India 

 Government Telegraph Department*. 



THE following experiment (or rather experiments) furnishes 

 a new means of demonstrating experimentally several laws 

 of vision, particularly such as relate to visual direction ; and, 

 suitably modified, may possibly be of some practical value. Pre- 

 pare a rather thick looking-glass by sprinkling its surface with 

 prepared chalk or other fine powder, shaking off the superfluous 

 powder so that the glass be covered with a thin coating of pow- 

 der, through which the reflected image of the face may be seen 

 with tolerable distinctness. If the glass so prepared be held 6 

 to 8 inches from the face of the observer, and so that he can see 

 a reflection of his own features, on shutting one eye the powder 

 will no longer appear equally distributed, but will appear arranged 

 in lines radiating from a point on the glass in front of the reflected 

 image of the pupil of the eye used in observing the phenomenon. 



* Communicated by the Author. 



