2 IMPORTANCE OF GOOD SPECIMENS. 



the conclusions of one observer should be strangely at 

 variance with those of another. Like the living 

 things of which it treats, physiology is incessantly 

 changing*, and the true physiologist endeavours not 

 only to add to existing knowledge, but so to add that 

 further changes, which he knows are inevitable, may 

 be made with the least possible derangement. In 

 progressing, he desires to provide for further, and he 

 hopes unceasing, progress. 



2. Importance of good specimens. Few things are 

 more difficult than to observe and interpret correctly 

 the mere structure of the tissues of man and animals. 

 It is, therefore, all-important to obtain specimens 

 which shall demonstrate clearly any new facts we 

 think we have proved. Every effort has been made 

 to illustrate by specimens the views I shall advance 

 in these lectures ; but the difficulties in the way of 

 obtaining and preserving good preparations, which 

 positively demonstrate at one view the facts upon 

 which conclusions concerning structure and growth 

 have been based, are so great that I scarcely think 

 my efforts can be attended with complete success. 



3. Magnifying powers employed. The specimens 

 which I shall have to describe have all been prepared 

 upon the same definite plan, and have been preserved 

 in the same medium. They have been examined under 

 objectives magnifying from 50 to nearly 3,000 diame- 

 ters.* In order that some idea may be formed of the 

 degree of amplification of the one-fiftieih object-glass 

 made for me by Messrs. Powell and Lealand in 1864, 

 I may be permitted to mention that if it were possible 

 to see a hair in its entire width under this power it 

 would appear to be nearly one foot in diameter, and 

 an object an inch in height would be made to appear 

 as if it was 250 feet high. 



4. Living and non-living. I shall have to direct 



* Proceedings of the Eoyal Society, January 19, 1865. " How 

 to work with the Microscope," 4th edition, p. 286. 



