The System of Saturn

[...net] of Saturn, which had been perceived by no one until now, and which we shall describe later, though it is not entirely free of error. From there, having indeed doubled the previous length Huygens is referring to the length of the telescope; doubling the length usually allowed for greater magnification and reduced chromatic aberration in this era., we were simultaneously made twice as close to the stars, and we noted all the phenomena much better and more easily. These tubes term: "tubes" (Latin: tubi). At this time, the word "telescope" was in use, but Huygens often uses "tubes" to refer to the physical instrument. of 23 feet were constructed from iron plating, and they have a glass lens inserted at one end, the width of which is four inches, but in which no circle opens wider than a diameter of two and one-third inches Huygens is describing the "aperture" of the lens—the usable area not obscured by the mounting or lens defects.. At the other end, which is naturally moved to the eye, there are two smaller glasses, equaling 1½ inches in diameter, joined together; in this way, they are equivalent to a single convex lens gathering parallel rays at an interval term: "interval" refers to the focal length. of 3 inches, or even a little narrower. From this, surely, the excellence of the larger glass The objective lens at the far end of the telescope. is to be estimated, as it is able to endure such a short convex lens A shorter focal length in the eyepiece increases magnification, but requires a very high-quality objective lens to maintain a clear image.. For it will be found demonstrated in our Dioptrics original: "Dioptricis." This refers to Huygens' mathematical study of the refraction of light and lens design. that the ratio of the image seen through the tube to that which is perceived by the naked eye, according to its diameter, is the same as the ratio of the focal distance in the outer glass to that which is in the inner or ocular glass.

By how much the things seen are enlarged by them.

It is certain, therefore, that this ratio in our telescopes is nearly a hundredfold, whereas the Galilean instruments did not proceed beyond thirtyfold.

How the magnification perceived by a telescope is estimated.

For we estimate the quantity of the increase in the same way as he [Galileo] did; namely, that each thing is said to be seen as much larger through the tubes than by naked sight as the angle of its extremities brought to the eye is larger, or as much wider as its image is painted on the back of the eye.

Another false estimation of the same.

There is, however, also another way of estimating magnification, but it has little truth, in which we determine the apparent size of something through a telescope without any consideration of the angle; as when we think the disk of Jupiter appears to us equal to a small circle of two or three fingers' width. But for that same circle, say of three fingers' diameter, to appear larger or smaller, it is ne—