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. 29. Q. What are some of the amazing discoveries made by the spectroscope within a few years? A. In chemistry it reveals substances never known before. It tells the chemical constitution of the sun, the movements taking place, the nature of comets, and nebulæ. 30. Q. By the spectroscope what do we know of the atmospheres of some of the other planets? A. We know that the atmospheres of Venus and Mars are like our own, and that those of Jupiter and Saturn are very unlike. 31. Q. From what are all our standards of time taken? A. From the stars. 32. Q. From what are the positions of the stars reckoned? A. As so many degrees, minutes and seconds from each other, from the zenith, or from a given meridian, or from the equator. 33. Q. How far apart are the stars called the Pointers in the Great Bear? A. Five degrees. 34. Q. To mistake the breadth of a hair, seen at a distance of one hundred and twenty-five feet, would cause how much of an error in the measurement of the distance of the sun from the earth? A. Three millions of miles. 35. Q. By means of a microscope how many lines ruled on a glass plate are we able to count within an inch? A. One hundred and twelve thousand. 36. Q. What angle does the smallest object that can be seen by a keen eye make? A. An angle of forty seconds. 37. Q. By putting six microscopes on the scale of the telescope on a mural circle, what degree of exactness are we able to reach? A. An exactness of one-tenth of a second, or one-thirty-six hundredth of an inch. 38. Q. In astronomical work how small measurements of time are made? A. To the minute fractional parts of a second. 39. Q. What is the personal equation of an observer? A. The time that it takes him to observe a thing and record it, which is substracted from all his observations in order to get at the true time. 40. Q. What is the parallax of a body? A. The angle that would be made by two lines coming from that body to the two ends of any conventional base, as the semi-diameter of the earth. 41. Q. What is the parallax of the moon, and also of the sun, with the semi-equatorial diameter of the earth for a base? A. That of the moon 57 seconds, and that of the nun 8.85 seconds.