ANATOMY AND PHYSIOLOGY 



only possible to understand disease by observing the 

 symptoms carefully at the bedside, and, if the disease 

 terminated fatally, by post-mortem examination, he was 

 so arduous in his pursuit of knowledge that within a 

 period of less than six months he had made over six 

 hundred autopsies a record that has seldom, if ever, 

 been equalled. Nor were his efforts fruitless, as a 

 single example will suffice to show. By his examina- 

 tions he was able to prove that diseases of the chest, 

 which had formerly been classed under the indefinite 

 name "peripneumonia," might involve three different 

 structures, the pleural sac covering the lungs, the lung 

 itself, and the bronchial tubes, the diseases affecting 

 these organs being known respectively as pleuritis, 

 pneumonia, and bronchitis, each one differing from the 

 others as to prognosis and treatment. The advantage 

 of such an exact classification needs no demonstration. 



LISTER AND THE PERFECTED MICROSCOPE 



At the same time when these broad macroscopical 

 distinctions were being drawn there were other workers 

 who were striving to go even deeper into the intrica- 

 cies of the animal mechanism with the aid of the mi- 

 croscope. This undertaking, however, was beset with 

 very great optical difficulties, and for a long time little 

 advance was made upon the work of preceding genera- 

 tions. Two great optical barriers, known technically 

 as spherical and chromatic aberration the one due to 

 a failure of the rays of light to fall all in one plane when 

 focalized through a lens, the other due to the dispersive 

 action of the lens in breaking the white light into pris- 

 matic colors confronted the makers of microscopic 



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