90 SCIENCE IN THE LAST HALF CENTURY. 



revealed the existence of rays having powerful chemical energy, or be- 

 yond the visible limits of either end of the spectrum ; while, to the 

 naturalist, it furnishes the means by which the forms of many highly 

 complicated objects may be represented, without that possibility of error 

 which is inherent in the work of the draughtsman. In fact, in many 

 cases, the stern impartiality of photography is au objection to its em- 

 ployment, — it makes no distinction between the important and the un- 

 important; and hence photographs of dissections, for example, are rarely 

 so useful as the work of a draughtsman who is at once accurate and in- 

 telligent. 



ASTRONOMY. 



The determination of the existence of a new planet, Neptune, far be- 

 yond the previously known bounds of the solar system, by mathematical 

 deduction from the facts of perturbation ; and the immediate confirniii- 

 tion of that determination, in the year 1846, by observers who turned 

 their telescopes into the part of the heavens indicated as its place, con- 

 stitute a remarkable testimony of nature to the validity of the principles 

 of the astronomy of our time. In addition, so many new asteroids have 

 been added to those which were already known to circulate in the place 

 which theoretically should be occupied by a planet, between Mars and 

 Jupiter, that their number now amounts to between two and three hun- 

 dred. I have already alluded to the extension of our knowledge of the 

 nature of the heavenly bodies by the employment of spectroscopy. It 

 has not only thrown wonderful light u[)on the physical and chemical 

 constitution of the sun, fixed stars, and nebulae and comets, but it holds 

 out a prospect of obtaining definite evidence as to the nature of our 

 so-called elementary bodies. 



ASTRONOMIC GEOLOGY. 



The application of the generalizations of thermotics to the problem of 

 the duration of the earth, and of deductions from tidal phenomena to 

 the determination of the length of the day and of the time of revolution 

 of the moon, in past epochs of the history of the universe; and the 

 demonstration of the competency of the great secular changes, known 

 under the general name of the precession of the equinoxes, to cause 

 corresponding modifications in the climate of the two hemispheres of 

 our globe, have brought astronomy into intimate relation with geol- 

 ogy. Geology, in fact, proves that in the course of the past history 

 of the earth, the climatic conditions of the same regions have been 

 widely different, and seeks the explanation of this important truth from 

 the sister sciences. The facts that, in the middle of the Tertiary epoch, 

 evergreen trees abounded within the arctic circle ; and that, in the long 

 subsequent Quaternary epoch, an arctic climate, with itsaccomi)animeut 

 of gigantic glaciers, obtained in the northern hemisphere, as far south 

 as Switzerland and central France, are as well established as any truths 



