206 APPENDIX. 



reversal of the image in the latter), and should during the course 

 learn the use of the camera lucida (Abbe's camera, of Zeiss, the 

 best). The stage-micrometer may also be examined at this time- 

 or later, and the student taught to prepare a scale (see Andrews) 

 bv drawing the lines, with camera, oil a card under different 

 powers (A + 2, D + 2, D -f- 4, of Zeiss), and labelling each 

 with the iiames of lenses and actual size of the spaces, as stated 

 on the micrometer. 



Pencil- drawing should begin as soon as the first specimen i& 

 in focus, and sketches should be made, from the very iirst exercise 



/ 



onward, of everything really studied. It is absolutely indis- 



t ~ / i/ 



pensable to keep a laboratory note-booh, which ought at any time 

 to give tangible evidence that the laboratory study is bearing 

 fruit ; and in the very first laboratory exercise a beginning should 

 be made in this direction. 



The preliminary microscopy of one or two laboratory peri- 

 ods, corresponding to the time spent in conferences upon the first 

 chapter of the text-book, leads naturally up to the easy micro- 

 scopical studies required in connection with the second chapter. 



CHAPTER II. (STRUCTURE OF LIVING ORGANISMS.) 



The laboratory work may be made very brief and simple,, 

 and the facts shown largely by illustration. The principal 

 organs of a plant and of a live or dissected animal may be shown, 

 and some of the more obvious tissues pointed out. A frog under 

 a bell-glass, and a flowering plant (geranium) in blossom, placed 

 side by side 011 the demonstration-table will serve to suggest 

 materials for the lists of organs and the comparisons called for. 



The skin of a Calla leaf is easily stripped off and demon- 

 strated to the naked eye as one form of tissue. It may then be 



' i< 



cut up and distributed for microscopic study and for proof that 

 it is composed of cells. (During this process air is apt to replace 

 water lost by evaporation, and must be displaced by alcohol, 

 which in turn must be removed by water.) 



For a first microscopical examination of tissue there is no 

 better object than the leaf of a moss (a species having thin broad 

 leaves should be chosen) or a fern prothallium. Other good 

 objects are thin sections of a potato-tuber from just Mow the 



