INTRODUCTION. XV 



all, as far as we have been able to find, that have been obtained in crystals. 

 Excepting hemoglobin, the crystallographic studies of proteins have been 

 very limited and inconclusive, and in so far as this substance is concerned 

 it is clear, from a study of the literature of the subject, that the inquiries 

 have with rare exceptions been of so superficial a character as to possess 

 little or no intrinsic value in indicating positive chemical differentiation. 

 Among the literature on hemoglobin we have found only very rare instances 

 where an adequate study was reported of the geometric characters of the 

 crystals, and we have failed to find any quantitative data of any value in 

 regard to the optical characters of the crystals in polarized light that might 

 be of service in showing zoological differentiations ; yet these very characters, 

 we believe, will be found to prove the best and most easily applied means 

 of differentiating the crystals of hemoglobin and of showing the identity 

 or non-identity of chemical composition or chemical constitution. The 

 comparative readiness with which hemoglobin can be crystallized, together 

 with the exceptional importance of this protein in animal life, led to its 

 selection as the subject of study. 



The important problems next demanding our attention were in regard 

 to the methods to be adopted to obtain graphic records of the crystals, 

 to the methods for preparing the crystals, and to the sources of supply 

 of the numerous and diverse kinds of blood required in order to yield 

 the necessary data. As to the first, experience has demonstrated that line- 

 drawings, lithography, perspective drawings, photomicrography, etc., each 

 has its advantages and disadvantages, yet it goes without saying that while 

 line-drawings are absolutely essential in the geometric descriptions of 

 crystals, the only means of reproduction which eliminates the personal 

 factor and gives at the same time a permanent and faithful record for veri- 

 fication and further study lies in the photomicroscope and its accessories. 

 The generally very poor reproductions of photomicrographs of crystals 

 that have appeared in print, together with the usual extreme unstability 

 of hemoglobin crystals, those of certain bloods melting at temperatures 

 scarcely above the freezing-point, seemed to us upon first thought to render 

 this method impracticable, except to a limited degree. Moreover, we 

 feared that, owing to the fact that the crystals and the solution in which 

 they have been formed are generally of so nearly the same color and tint, 

 satisfactory reproductions for printing would be found to be practically 

 impossible; but these difficulties we overcame. 



All of our photomicrographic negatives were made with ordinary 

 laboratory apparatus. We used a standard Bausch and Lomb microscope, 

 and almost without exception a 2-inch eyepiece and a objective, which 

 gave us a magnification in our negatives of about 250. Occasionally we 

 used higher powers, giving us a magnification of about 500, 800, and 1,200. 

 Many of our negatives are not up to the standard we sought, because of 

 the great sensitivity of many of the crystals to the slightest increase of 

 temperature during the focusing and exposure in the photomicrographic 

 apparatus, or to the little or no contrast in the color of the crystals and 

 solution, in many instances the crystals being discernible solely by the 



