466 Scientific Intelligence. [Jdne, 



ively examined the multiplication of images, and the beautiful 

 colours which accompany them, in some specimens of Iceland spar. 

 All these philosophers ascribe the phenomena tojissiires within the 

 crystal, and almost all of them explain the multiplication of the 

 images by internal reflections. Dr. Brewster has discovered the 

 true cause of all these appearances, and can communicate the 

 faculty of producing them to any piece of Iceland spar, or any 

 double refracting crystal. What was supposed to be an accidental 

 fissure, he has shown to be an interrupting stratum, either of the 

 same or of a different substance, crystallized in a different manner 

 from the rest of the mineral, and producing the complimentary 

 colours by depolarizing two of the images ; and he has found that 

 this stratum is always perpendicular to the shorter diagonal of one 

 of the rhomboidal faces. The artificial rhomboids which have been 

 constructed upon this principle imitate, in the most perfect manner, 

 all the phenomena which appear in the real crystal. 



VII. Optical Properties oj Iodine. 



In attempting to obtain a measure of the refractive power of 

 iodine by ascertaining the angle at which it polarizes light, Dr. 

 Brewster found that it possessed the property of polarizing the light 

 which it reflected in two opposite planes, a property which is pecu- 

 liar to metallic bodies. 



VIII. Queries respecting the. Visihiliiy of the Stars in the Day-time. 



(To Dr. Thomson.) 

 SIR, 



Having for some time past made a considerable number of obser- 

 vations on the celestial bodies in the day-time, the following con- 

 clusions, amongst many others, have been established, from a great 

 number of observations and experiments, viz. that in every instance 

 - an increase of the magnifying power of the telescope has the prin- 

 cipal effect in rendering a star easily perceptible ; that the diminu- 

 tion of the aperture of the object glass produces a very slight elfect, 

 in some cases none at all, and that when the aperture is contracted 

 beyond a certain limit it produces a hurtful effect, and even prevents 

 the object from being distinguished. These conclusions, 1 have 

 reason to believe, coincide with the deductions of Mr. Short and 

 others who have made similar observations ; so that it may be con- 

 sidered as a fact sufficiently established, that magnifying power is 

 requisite for distinguishing a star in the, day-time, and that the more 

 the magnifying power is increased, the more splendid and brilliant 

 the star appears. 



Query : What is the cause why the magnifying power of telescopes 

 produces this effect ? 



In regard to the planets, a probable solution might be given from 

 the consideration that the telescope, by enlarging the angle of vision, 

 augments the apparent size of tlie object, thus presenting a larger 

 surface to the eye, which is partly iho reason why the moon is 



