INTRODUCTION TO THE ATLAS 



As has been stated in the Preface, the original figures in the Atlas are reproductions of 

 actual objects by the three-color photographic process, without retouching of the process-plates. 

 Figures and letters are not used to designate the special histological structures. I have indi- 

 cated, however, as clearly as I could without such aid, what I specially desire to illustrate in 

 each picture. Most of the figures it has been easy to describe; but others will be found to 

 require careful and patient study, particularly those relating to embryology. In the study 

 of these pictures, it will be of advantage to consult first the 'descriptions and illustrations which 

 latter are more or less diagrammatic given in the text. It should then be easy to recognize the 

 structures as they are shown in the Atlas. Indeed, I have endeavored to supplement the text 

 with what approaches actual laboratory study. The hours spent in such work, although without 

 the advantage of a microscope and personal demonstration, can hardly fail of practical value. 



It will be evident to histologists that I have been peculiarly fortunate in certain objects for 

 reproduction. The section of the stomach, showing peptic cells and acid-cells, I believe to be 

 unique ; the platelets from human blood are seldom so clearly shown ; the same may be said 

 of branching heart-muscle ; serous and mucous cells and demilunes of the submaxillary ; intes- 

 tinal glands ; islands of Langerhans ; solitary glands ; structure of adrenals, spleen, thyroid, 

 and thymus ; bone-canaliculi and elastic cartilage. The sections of the human embryo are 

 seldom seen and will repay careful examination. 



As a matter that can not fail to be of interest, I add to this introduction a note on tech- 

 nique, by Dr. Learning : 



NOTE ON THE TECHNIQUE OF PHOTOMICROGRAPHY IN COLORS 



As at the present time photomicrography finds one of its most useful -applications in 

 medicine, it seems but natural to expect that the future photomicrographer should be found 

 among medical students. Bearing this in mind, a few words on the photomicrographs in color 

 in the Atlas may prove of interest. The apparatus need not be discussed ; any first-class instal- 

 lation will be quite equal to the demands upon it ; but it is essential that the operator should 

 be thoroughly familiar with his apparatus and its peculiarities. The most important of the 

 accessories will be the color-filters, or screens, whereby the rays not needed are absorbed before 

 they reach the object, and only those desired for each particular negative are allowed to pass 

 through. It must be remembered that for our purposes the visible spectrum is divided arbi- 

 trarily into three zones ; one near the red end, one near the blue end and one near the middle. 

 From each of these zones one color is taken, and these three colors form the primary, or print- 

 ing colors; they are a particular pinkish or madder-red, a canary-yellow, and a brilliant greenish 

 blue. If we combine the above in pairs, we obtain an entirely new set of colors, and these form 

 our taking screens, or color-filters. These new colors are termed the secondary colors. Thus, 

 primary red plus primary yellow makes secondary red an orange-red screen which the 

 spectroscope will show to admit the red end of the spectrum only ; primary red plus primary 

 blue equals secondary violet blue end only; and finally, primary blue plus primary yellow 

 makes secondary green middle only. It will be seen from the above, that these secondary 

 colors are complementary to the primary. For example, an object taken through the green 

 screen (blue and yellow) is printed in red ; the violet screen (blue and red) is printed in 



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