258.S Journal of Applied Microscopy 



to republish the method in order that the readers of this Journal may have the 

 benefit of it. Schultze himself says regarding it that " it is the most elegant and 

 best method for studying the macroscopic phenomena in the formation of bone, 

 and frequently makes superfluous the tedious method of serial sections." 



Embryos should be hardened in alcohol for at least X days. (Embryos fixed 

 and hardened in aciii solutions cannot be used in this method.) From alcohol 

 specimens are transferred directly to an aqueous solution of KOH of a concen- 

 tration of from 3 to 5 per cent. In case the embryos are large the brain should 

 be taken out through the great fontanelles with fine forceps and needles, and 

 the thoracic and abdominal viscera should be removed through a median 

 longitudinal incision along the ventral body wall. After a period of time which 

 varies in length according to the size of the specimens, the duration of the alco- 

 hol hardening and the concentration of the KOH, the embryos become trans- 

 parent. They may be then permanently preserved in the following solution : 

 Water ------ 100 parts 



Glycerine ----- .30 parts 



Formal (3.5 per cent.) ... i> parts 



In these preparations every point where ossification has taken place appears 

 ivory white in color and opaque, while all other portions are transparent or trans- 

 lucent. R. P. 



Question : " Why do Some Crystals Polarize, and Some Not, 

 e. g.. Common Salt, etc. ?" 



A ray of white light is a complex bundle of waves of varying forms and 

 rapidity of motion. 



Polarization by a crystal or other substance is merely the revelation to the 

 eye, by colors, through the use of a pair of nicol prisms as a test agency, of dis- 

 turbance so affected in the ray of white light, during its transmission through the 

 particular substance. When such a ray is passed through a transparent sub- 

 stance, its path may be broken (simply repeated), but the ray itself may pass 

 otherwise entirely undisturbed through certain substances. In such cases, the 

 substance among whose molecules the ray passes unaffected is termed " simply 

 repeating"; merely its path broken and no colors of polarization produced 

 (e.g., air, water, glass, common salt, garnet, and all substances crystallizing in the 

 Isometric system, under ordinary conditions of pressure). 



Nor is the ray even affected by " doubly refracting substances " (e. g., all 

 other crystals but those of the Isometric system), when passed through them in 

 certain definite crystallographic directions, called the "optic axes." In neither 

 of these cases do colors of polarization appear, when the ray is tested between 

 nicols whose planes are crossed, i. e., producing a dark field. 



On the other other hand, when the ray passes through " double refracting " 

 crystals in any other directions than those of the " optic axes," or through any 

 transparent substances whatever, when these happen to be under strain, or 

 through transparent substances of finely laminated or finely radial structure 

 (e. g., sections of pearl, opal, chalcedory, ruled gratings on glass, etc.), then also 

 that disturbance of the ray occurs, by which it is broken up into waves of vary- 

 ing intensity, whose presence will be revealed as " polarization " colors, when 

 tested between crossed nicols. 



However, exactly how the molecules of certain substances, or within certain 

 structures, produce this disturbance in a transmitted ray — or even how they pro- 

 duce the familiar disturbance in a reflected ray, called "color" — we cannot 

 explain, as long as the very forms and the modes of optical action of the infinitely 

 minute molecules themselves remain beyond our ken. 

 Columbia University. Alexis A. JULIEN, 



