259. How may you see that light moves in straight lines? State various familiar recognitions of this fact. Illustrate the fact that the intensity of light is inversely as the square of the distance. 260. What is said of the velocity of light in regard to ordinary distances? How long is light coming from the sun to the earth? What is said of the light coming to us from certain stars? 261. Give the observation of Roemer represented in Fig. 226. 262. What is said of the reflection of light? What of its reflection in relation to seeing? What of the images formed in mirrors? 263. Show by Fig. 228 why the image in a mirror seems to be as the same distance behind it that the object is before it. Explain by Fig. 229 the operation of the kaleidoscope. 264. Explain the operation of a concave mirror by Fig. 230. Explain that of a convex mirror by Fig. 231. 265. What is meant by the refraction of light? Illustrate its reflection in passing from a denser into a rarer medium. Then from a rarer into a denser. 266. How is the refraction in regard to a perpendicular in the two cases? Explain dawn and twilight. Explain what is represented in Fig. 234. 267. What are mirages? Describe the mirage which occurred at Ramsgate. Describe that seen by Captain Scoresby. Relate the incident which occurred at New Haven. 268. What is said of mirages in deserts? 269. Describe the mirage of the French coast. Explain what is meant by the visual angle as illustrated by Fig. 236. 270. Explain Fig. 237. What are lenses? What are the different kinds? 271. What is the difference of effect in convex and concave lenses? Explain the effect of a convex lens on the visual angle. What is said of microscopes and telescopes? 272. Describe and explain the magic lantern. Describe and explain the camera obscura. 273. Describe the arrangement of a camera for sketching. How is the eye like a camera? 274. Describe the arrangement of the parts of the eye as mapped in Fig. 244. 275. Show now particularly how the eye is like a camera. What is said of the influence of the cornea on the light? Show what is required for distinct vision, as illustrated in Fig. 245. Show why it is that objects brought very near the eye are not seen distinctly. 276. What is said of the microscope? Explain the difficulty in the near-sighted. In the far-sighted. 277. How can you show that the images of objects in the retina are inverted? Give in full what is said of explanations of the fact that we see objects erect notwithstanding this inversion. Explain single vision. 278. By what simple experiment can you show the explanation of single vision to be correct? What is said of squinting? Explain the stereoscope. 279. What is said of distinct impressions on the retina? Explain the thaumatrope. 280. State in full what is said of the compound nature of light. Give the proportions of the colors in it. What is said about there being only three colors?