19. Q. What is probable as to the temperature of Mars? A. It is probable that Mars, that receives but one-quarter as much heat as the earth, has a temperature as high as ours.
20. Q. What two radically different kinds of telescopes are made? A. The refracting telescope, and the reflecting telescope.
21. Q. Why is the refracting telescope so called? A. Because it is dependent on the refraction of light through glass lenses.
22. Q. Why is the reflecting telescope so called? A. Because it acts by reflecting the light from a concave mirror.
23. Q. What is the loss of light in the use of each kind of telescope? A. In passing through glass lenses it is about two-tenths. By reflection it is often one-half.
24. Q. In view of this peculiarity, among others, what is held as to the comparative quality of the two kinds of telescopes? A. That a twenty-six inch refractor is fully equal to any six-foot reflector.
25. Q. What is the weight of the Lord Rosse reflecting telescope? A. It has a metallic mirror weighing six tons, and a tube forty feet long, which, with its appurtenances, weighs seven tons more.
26. Q. What is a spectrum? A. A collection of the colors which are dispersed by a prism from any given light.
27. Q. If the light is sunlight what is the spectrum called? A. A solar spectrum.
28. Q. What is a spectroscope? A. An instrument to see these spectra.