BIOLOGICAL APPARATUS. 323 



much confidence as was placed in those of a fourth that amount 

 forty years ago. 



Numerous examples of microscopes of the best modern con- 

 struction are to be seen in the collection, and the contribution 

 which the President of the Microscopical Society has made to 

 this Handbook gives a full account of the principles by which the 

 makers of these exquisite instruments are guided. 



Modern histology could hardly have existed, in any shape, with- 

 out the modern microscope, inasmuch as the meaning of many 

 optical appearances of animal and vegetable structures becomes 

 apparent only under the high magnifying powers and perfect defi- 

 nition of our present instruments. But the precise and definite 

 form which our notions of structure and development have been 

 acquiring during the last ten or fifteen years, is mainly due to the 

 fact that the anatomist has been supplied by the chemist with com- 

 pounds such as chromic acid, perosmic acid, picric acid, and the 

 like, by which soft organic bodies can be rendered hard enough to 

 be cut into the thinnest slices without alteration of their essential 

 form and arrangement, and by which different elements of the 

 tissues can be made to assume different colours and thus become 

 readily distinguishable. 



Hence have arisen various modes of preparing, staining, and 

 slicing organic structures, and many different kinds of instruments 

 adapted for the latter purpose. Very perfect methods of preserving 

 the most delicate microscopic objects have been invented, and the 

 art of injection has been immensely improved both in the complete- 

 ness of its results and the certainty with which they can be obtained. 



In many cases, it is of importance to be able to watch the 

 effects of the application of heat and cold to living objects, and 

 to be able to follow the development of the same microscopic 

 organism for a long period, under conditions which prevent the 

 intrusion of other organisms. Hence the invention of hot and cold 

 stages, of moist chambers, cultivation apparatuses and calorific 

 tables, of which examples are exhibited. 



Y 2 



