14 EIGHTH REPORT — 1838. 



in ph3'sical optics, I may have another opportunity of explaining. At 

 present, I beg the attention of the meeting to another part of the ex- 

 periment. We have seen that the effects of interference are distinctly 

 developed in a certain position of the retarding plates. This position, 

 when the effects are most distinct, is that in which the edges of the 

 plates are turned towards the red end of the spectrum and are parallel 

 to its fixed lines. If we give the plates a motion of rotation in their 

 own plane, the bands and lines and the phenomena of absorption be- 

 come less and less distinct as the angle between the edges of the plates 

 and the lines of the spectrum increases. When this angle is 90° the 

 bands disappear altogether, and during the next 90° of rotation they 

 continue invisible. At 270° of azimuth they begin to reappear, and 

 attain their maximum distinctness at 360°, when they have returned to 

 their original position. Here then we have certain phenomena of in- 

 terference, and also of absorption, distinctly exhibited when the least 

 refrangible side of the retarded ray is towards the most refrangible 

 side of the spectrum, or towards the most refrangible side of the unre- 

 tarded ray ; while the same phenomena disappear altogether when the 

 most refrangible side of the retarded ray is towards the least refrangible 

 side of the unretarded ray ; and between these two opposite positions 

 we have phenomena of an intermediate character. Hence I conclude, 

 that the different sides of the rays of homogeneous light have different 

 properties when they are separated by prismatic refraction or by the 

 diffraction of grooved surfaces or gratings, — that is, these rays have po- 

 larity. W^hen light is rendered as homogeneous as possible by absorp- 

 tion, or when it is emitted in the most homogeneous state by certain 

 coloured flames, it exhibits none of the indications of polarity above 

 mentioned. The reason of this is, that the more or less refrangible 

 sides of the rays lie in every direction, but as soon as these sides are 

 arranged in the same direction by prismatic refraction or by diffraction, 

 the light displays the same properties as if it had originally formed part 

 of a spectrum." 



On some Preparations of the Eye by Mr. Clay Wallace, of New 

 York. By Sir D. Brewster. 



Sir David Brewster laid before the Section a series of beautiful pre- 

 parations of the eye made by Mr. Clay Wallace, an able oculist in 

 New York, calculated to establish some important points in the theory 

 of vision. Mr. Clay Wallace, he stated, considers that he has discovered 

 the apparatus by which the eye is adjusted to different distances. This 

 adjustment is, he conceives, effected in two ways. In eyes which have 

 spherical lenses it is produced by 2i falciform, or hook-shaped muscle, 

 attached only to one side of the lens, which by its contraction brings 

 the crystalline lens nearer the retina. In this case, it is obvious that 

 the lens will have a slight motion of rotation, and that the diameter, 

 which was in the axis of vision previous to the contraction of the mus- 

 cle, will be moved out of that axis after the adjustment, so that at difr 



