LENSES. 19 



know in order to work any of them well, and they are 

 not likely to become disarranged. 



The course of experiments to be given will be gen- 

 erally adapted to both the porte lumiere, and the lan- 

 tern, but the adjustments will be described for the 

 former. If, however, some special arrangement of the 

 lantern will be needed for a given experiment, it will 

 be pointed out, 



A sufficient number and variety of experiments in 

 physics, chemistry, and natural history, will be de- 

 scribed, to make any one who is practically interested 

 in the work so familiar with the apparatus and its 

 working, that he will need no further instruction in the 

 art of projecting. 



LENSES. 



All the lanterns in the market are furnished with the 

 proper lenses for the ordinary kinds of projections, 

 such as transparencies, etc. Solar microscopes are also 

 generally well provided with ler.ses suitable for the work 

 to be done with them, but as this treatise is for the use of 

 those who have neither, yet who would like to experi- 

 ment, some things, necessary to be known about lenses, 

 and their uses are mentioned here, chiefly as to their 

 adaptability to the experiments with the porte lumiere. 

 In Fig. 1 1 the rays of light are shown to fall upon the 



