24 The Structure of Protoplasm 



fixation reveals their presence (16). The nuclei of many other 

 animal cells are optically empty (7) , yet it would be foolhardy to 

 claim to a geneticist that no genie arrangement or differentiation is 

 present. Many other such cases in which the refractive indices of 

 cell constituents are too much alike to form visible edges can be 

 cited. 



DOUBLE REFRACTION OF PROTOPLASM 



Fortunately, there are other approaches to the problem than by 

 the use of the microscope alone. Of these, polarized light has been 

 employed with considerable success.^ It has long been known that 

 muscle, nerve, sperm, and certain other specialized cells are doubly 

 refractive (41) . Likewise, chromosomes, chloroplasts, and other cell 

 constituents may exhibit this property (41) . The question arises: 

 Is this birefringence due to the presence of anisotropic particles 

 (intrinsic double refraction) or is it produced by the regular arrange- 

 ment of isotropic rods or plates in an isotropic medium of different 

 index of refraction (form double refraction) ? It follows from the 

 theory of Wiener that the presence of form birefringence in an object 

 can be detected by varying the refractive index of the medium 

 bathing the particles, whereupon the magnitude of the double refrac- 

 tion changes, until, at a certain refractive index, the object appears 

 isotropic, i. e., dark in all positions between crossed nicols. On 

 raising the refractive index of the medium beyond this point, the 

 form birefringence reappears. If, however, the object is intrinsically 

 anisotropic, this disappearance of birefringence does not take place, 

 and the extent of the double refraction does not depend on the 

 refractive index of the medium. Upon the application of this test to 

 intracellular structures, it has been found that, although form 

 birefringence (generally rod birefringence) may be present, some 

 intrinsic double refraction may be superimposed on it. Under such 

 conditions, the curve of double refraction of the object vs. refractive 

 index of the medium reaches a minimum but does not drop to zero. 

 Since, in general, it is not possible to carry out this procedure on 

 living material and since one must assume that the imbibing 

 medium does not interact with the micells, its use with protoplasm 

 is limited. At all events, when an object appears bright between 

 crossed nicols it is clear that oriented anisodiametric particles are 

 present. 



Although chromosomes, chloroplasts, and other specialized struc- 

 tures exhibit definite birefringence, attempts to detect double refrac- 



