i 4 8 G. L. Kite 



entirely beyond the focal plane." Such facts should serve to make 

 it evident that one can easily fall into error in interpreting the optical 

 image of a living cell. Porter 12 recommends " a working knowledge 

 of the phenomena and laws of diffraction," as a safeguard against 

 this form of error. 



The third and by far the most important source of error is due 

 to the peculiar and little known optical properties of living matter. 

 The phenomena of reflection, refraction, absorption, dispersion, inter- 

 ference, diffraction 13 and a scattering action on light 14 are all exhib- 

 ited by this substance, with the result that a correct interpretation 

 of the image of a living cell is frequently impossible. Further- 

 more, many cells are so opaque and turbid that the interior is not 

 visible. Cloudiness or turbidity is almost a universal property of 

 protoplasm and appears to be due chiefly to dispersion, refraction, 

 diffraction, and the scattering action on light of the colloidal par- 

 ticles which may be considered as the real structural units of all 

 protoplasm. 



Globules, granules and cell walls frequently show diffraction 

 halos that are difficult to interpret in undissected cells. 



The aim of this investigation is to determine the physical state 

 and the molar structure of protoplasm. The methods are radically 

 different from those heretofore used, and are believed to be adequate 

 for this purpose. Dissection and vital staining are used to deter- 

 mine the truthfulness of the optical image and the actual structure 

 of cells. Unfortunately, the amount of the error involved in the 

 employment of these methods depends entirely on the skill of the 

 experimenter; but it is believed that the error becomes quite small 

 with complete mastery of the methods. 



The structural changes that cells may undergo during the time 



13 Excellent expositions of the principles of physical optics are given by: 

 WOOD, R. W: 1911, Physical Optics; DRUDE, P.: 1912, Lehrbuch der Optik; 

 PRESTON: 1901, Theory of Light. 



14 Lord Rayleigh (Philosophical magazine, xli, p. 107) has pointed out that 

 reflection and refraction have no application unless the surface of the disturbing 

 body is larger than many square wave-lengths. The turbidity of protoplasmic 

 sols, then, is due entirely to the scattering action on light of the minute aggre- 

 gates of the disperse phase, while reflection, refraction, diffraction, dispersion 

 and a scattering action on light are all seemingly involved in the production of 

 turbidity by gels. 



