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.