MONOSACCHARIDS OR GLUCOSES. 89 



Common light is due to vibratory disturbances in the ether, 

 which are propagated through it as waves, the direction of the 

 vibrations being transverse to that of propagation. In all places 

 where light is polarized its vibrations, still transverse to the direc- 

 tion of the ray, are all in one plane. Light may be polarized by 

 transmitting it through most crystals, and if it is then transmitted 

 through another crystal it will be observed that when this is in 

 certain positions it will pass most easily, and that in positions at 

 right angles to these it will be quenched entirely. It is supposed 

 that the molecular structure of these crystals is such as to make 

 them transparent for vibrations in one plane and opaque to those in 

 the plane at right angles. The rotation of the plane of polarization 

 by passing the polarized light through a crystal constitutes rotary 

 polarization, and is the principle upon which the polariscope is 

 constructed. 



The crystal which polarizes the light is the polarizer, and that 

 which distinguishes it is the analyzer. 



This power to rotate the plane of polarization is possessed by 

 other substances than crystals, such as solutions of various sub- 

 stances, among them being sugar ; and as each of these substances 

 rotates the plane through a different number of degrees of a circle, 

 this fact enables the investigator to determine with what substance 

 he is dealing. Substances having this power of rotating the plane 

 of the polarized ray are said to be optically active; and those 

 which rotate it to the right are dextrorotatory, and those that 

 rotate it to the left, levorotatory. Inasmuch as the rotation is 

 different for each of the component parts of white light, this kind 

 of light cannot be used, but in its stead light of a single color, 

 monochromatic light, must be used. This is usually the yellow 

 light produced by burning a salt of sodium in the flame of a 

 Bunsen burner. 



A polariscope or polari meter which is specially adapted to the 

 estimation of the amount of sugar in a given solution is called a 

 saccharimeter. Of these, there are various kinds, the one most 

 commonly used being Laurent's. 



Laurent's Polarimeter. This and its use are described by Prof. 

 Bartley in his Medical Chemistry in the following language : 

 " Laurent's polarimeter (Fig. 82) is one of the simplest and best. 

 In this instrument one-half of the field of vision is covered by a 

 very thin plate of quartz, which slightly rotates the plane of the 

 light passing through it, and causes some light to pass even when 

 the polarizer and analyzer, both of which are Nicol prisms, are 

 crossed. If the analyzer (h) is rotated so as to cause the quartz 

 plate to become dark, the light passes through the uncovered half 

 of the field. In an intermediate position the two halves of the field 

 appear equally illuminated. The scale (c) is so graduated that 

 this position of the analyzer is made the zero point of the instru- 



