PREFACE vii 



of M. Ducos du Hauron ; but the cost of reproduction in this way is 

 prohibitory as regards use in ordinary text-books. For several years 

 before 1895, the printing, from ordinary process-plates, of reproductions 

 of colored drawings or lithographs had been accomplished commer- 

 cially. So far as I know, however, this has not been done before in 

 photomicrography. The difficulties encountered in these efforts were 

 considerable ; but they were at last overcome by the skilful and patient 

 cooperation of Dr. Learning and of the American Colortype Company. 

 Forty-eight illustrations of this kind are in the Atlas at the end of the 

 book. With almost perfect fidelity as regards definition and color, they 

 reproduce the objects used, and they may be studied as one would study 

 with the microscope actual preparations. Of necessity, however, each 

 picture represents a single field focussed for but one plane. The pic- 

 tures taken from actual objects are supplemented in the Atlas with 

 reproductions, by the same process, of plates selected from the histo- 

 logical atlas of Sobotta. These, though somewhat idealized, are intro- 

 duced for the reason that they represent difficult objects, and some 

 illustrate in a single page groups of objects that would require many 

 more figures if made from actual specimens. Four figures, showing 

 the first stages of cleavage of the ovum, have been reproduced, without 

 reduction in size, from the original negatives taken by Dr. Learning for 

 Professor Wilson's Atlas. These are printed in blue, although the 

 original prints were in black and white. 



It seemed to me very important to solve the problem of reproducing, 

 at a moderate cost, stained objects in colors, for the reason that photo- 

 micrographs in black and white are nearly always inadequate and some- 

 times misleading. Compare, for example, Fig. 102, p. 464, in the text, 

 with Fig. 4, Plate V, in the Atlas, both showing sections of the pancreas. 

 For useful illustration, indeed, color-figures must be used, in the absence 

 of study of actual objects, if for no other reason than that histological 

 elements are so often differentiated by their affinities for various dyes 

 and can be recognized only in stained preparations. Again, the student 

 seldom seqs in his laboratory work the appearances represented in books, 

 even by the best and most costly colored drawings. 



The technique employed in the photographic work is given by Dr. 

 Learning, in a note introductory to the Atlas. The negatives, taken 

 through color-filters, were given, with the objects, to the Colortype Com- 

 pany, and put into the hands of their most intelligent workman, who 

 was instructed first in the use of the microscope. The color-plates were 

 made through a transparent screen with a ruling of one hundred and 

 seventy-five lines to the inch. The etching was done in comparison 

 with the actual objects, which were used as color-guides. This was the 



