370 Intelligence and Miscellanies. 



"is 



The term polarisation thus derived from the corpuscular 

 theory of hght, has also been adopted as a convenient term 

 by those who support the theory of undulations. 



Light is also found to be polarised by transmission through 

 some transparent bodies, in the same manner as it is by re- 

 flection from others. And when light so polarised is made 

 to pass through transparent crystallized substances, some of 

 the most splendid phenomena in optics are produced. 



The small instrument which is represented in the annex- 

 ed Plate, will exhibit many of these phenomena, and will 

 enable the enquirer in this branch of science to make new 

 experiments. 



Fig. 1st. shews the instrument when put together for use. 



Fig. 2d. shews the principal parts separately, the one 

 marked a. the other 6. 



Fig. 3d. is the part contained within h that is moveable by 

 means of the small external pin ?', and to which is attached 

 the revolving plate d. 



When the instrument is to be used, the two parts a and 6, 

 fig. 2, must be placed as they arc represented in fig. 1, and 

 the lines marked c in the figures 1 and 2 are alivays in the 

 first instance to be made to coincide. The pin of the move- 

 able piece, fig. 3, should also be brought home to the line at 

 c. This position however of the pin, and the relative posi- 

 tions of the lines c, will be continually changing while the in- 

 strument is in use. 



The small revolving plate d has one hole uncovered and 

 five covered with different bodies ; these are attached with 

 cement, and may be removed if the holes they cover should 

 be wanted for other experiments. 



The phenomena of polarisation may be conveniently ob- 

 served in the following order : 



Let the uncovered hole of ^be called hole 1, the next long- 

 est hole 2 ; and so on. By means of the pin on the revolv- 

 ing plate d bring hole 1 over the aperture in the centre of 6, 

 and then place a in h as before directed. Holding the in- 

 strument now between the eye and the light, look through the 

 hole at the bottom of b and very little light will be perceived. 

 But if while the instrument is held to the eye the tube a be 

 gradually moved round so as to bring the line at c to 

 coincide successively with the other lines on ft, more light 

 will gradually appear, and when a has been moved through 

 a quarter of the circle the light will be at its maximum. If 

 a be further turned round the light will now decrease, and 



