INTRODUCTION. 



either physical astronomy or physiology, who have never yet 

 looked through a telescope or microscope ; but certainly new worlds 

 to all those who lived two centuries ago. Before Galileo's time 

 what unknown wonders in the depths of the sky, in the world of 

 the infinitely great ! What astonishing revelations in the world 

 of the infinitely little, since Swammerdam ! New sciences have 

 sprung up which would not have been possible without the help of 

 these powerful means of investigation placed by optics at the 

 disposal of observers. Thanks to the microscope, the structure 

 of the animal and vegetable tissues, the most capable of disclosing 

 the mechanism of life, is known in its most minute details. By 

 means of the telescope the eye penetrates into infinite space, and 

 there discovers millions of stars, the existence of which the eye 

 could scarcely suspect at such enormous distances, that it takes 

 centuries and thousands of centuries for their light to reach us, 

 although the light waves travel with prodigious velocity through 

 the ether. 



Nevertheless there is nothing more simple than the manufacture 

 of these optical instruments, nothing more easy than to understand 

 their principles and to explain their effects, and, lastly, nothing 

 easier, with patience and study, than to acquire the practical 

 knowledge necessary to their fruitful uses. 



Other instruments, based on similar principles, such as helio- 

 stats, sextants, goniometers, then spectroscopes and apparatus for 

 lighthouses, are employed for scientific researches of different kinds 

 and render precious service in astronomy, mineralogy, and travels ; on 

 all accounts they deserve to be described and studied. The siderostat, 

 an invention due to Hooke, Laussedat and Foucault, although it has 

 not yet come into general use, must be referred to for the great 

 help it is destined to render in the researches of physical astronomy, 

 for instance, in the study of solar phenomena. 



But one of the most interesting applications of the properties 

 of light an invention still recent and already brought to a rare 

 degree of perfection is that which allows us to reproduce instan- 

 taneously, and with wonderful fidelity, all objects illuminated by 

 a sufficiently intense light source. In the present day photo- 

 graphy is a popular art, popular in its processes and results, but 

 none the less interesting in its principles and method, nor 



