150 



Salmonidse may have its use, inasmuch as the opaque white ova are 

 more conspicuous than the transparent, — the dead than the living, — 

 and in consequence, the one may serve as lures and divert from the 

 others the many enemies to whom they are attractive food. 



February 12, 1852. 

 COLONEL SABINE, R.A., V.P. and Treas., in the Chair. 



The following communications were read : — 



1. The subjoined Letter from Professor Haidinger to Captain 

 Smyth, R.N., For. Sec. R.S., dated Vienna, January 15, 1852. 

 Received January 23, 1852. 



Sir, — The great success with which optical researches are treated 

 of in the publications of the Royal Society, must make me anxious 

 to lay before the Society, in a few words, a concise and convincing 

 demonstration of the theorem that in a ray of polarized light the 

 vibrations are perpendicular to the plane of polarization, conform- 

 ably to the views of MAL Fresnel and Cauchy, and not in the plane 

 of polarization, as some other mathematicians have maintained. 



My demonstration is founded on the nature of dichroitic crystals, 

 as tourmaline, sapphire, idocrase, &c. Any perfectly homogeneous 

 crystal of this description presents two different tints of colours. 

 One of them appears in the direction of the axis, as well as in all 

 directions perpendicular to it, and it is always polarized in a plane 

 passing through the axis ; the other tint appears in every azimuth in 

 the directions perpendicular to the axis, and it is polarized in a plane 

 perpendicular to the axis. The latter of these colours does not 

 appear at all, if the crystal is examined in the direction of the axis ; 

 if it depend at all on transverse vibrations, all vibrations of this kind, 

 transverse or perpendicular to the axis, are at once excluded, and the 

 only vibrations that can possibly belong to the colour of the extra- 

 ordinary ray produced in the crj^stal, are those parallel to the direc- 

 tion of the axis. But agreeably to observation the plane of polariza- 

 tion is itself perpendicular to the axis, the vibrations therefore take 

 place in Airec'doTi^ perpendicular to the plane of polarization. 



Trichroitic crystals of course will yield a similar demonstration, 

 as cordierite, andalusite, diaspore, axinite, and others. 



I shall not fail to send a copy of the communication I am to pre- 

 sent today to the Vienna Academy, as soon as it shall have been 

 printed. 



The importance of the subject will, I am confident, plead as an 

 apology for my trespassing on your kindness in thus making the 

 request, that you will lay the present communication before the 

 Royal Society. 



I have the honour to be. 

 My dear Sir, 



Your obedient Servant, 



W. Haidingek. 



