Note 57 — Chapter XV · THE WONDERS OF THE REFRACTION OF LIGHT (continued).

The arrangement of lenses, above described, constitutes the principle of what is termed the “astronomical telescope;” for this makes the objects appear upside down. But, though the inversion of a star or planet is a matter of no moment in astronomical observation, such an effect is most disagreeable when applied to terrestrial objects. The ordinary telescope for land purposes, therefore—or the “day telescope,” as it is usually styled—has two other lenses behind the eye-lens. These lenses have both the same focus as the eye-lens itself, and are placed at a fixed distance from each other, such distance being equal to the sum of their focal lengths: that is to say, if the eye-lens have a focus of 2 inches, then each of the two other glasses should have the same length of focus, and be placed at 4 (2 + 2) inches apart from one another. The magnifying power of the day telescope may be calculated in the same manner as that of the astronomical one above explained; for the two additional lenses in the day instruments, having the same focal length as the eye-lens itself, produce no further enlargement of the objects, but serve only to cross the rays a second time, and so to render the image erect instead of inverted.