Two famous statements made by William Herschel underscore the importance of this lengthy chapter as preparation for observing the Herschel objects. One is that "Seeing is in some respect an art, which must be learnt." As described below, there is no doubt that the human eye can be trained to see better in at least four distinct areas involving the viewing of celestial wonders - these being dark adaptation, averted vision, color perception, and visual acuity. And the reason that this is really possible is that the eye works not alone but in conjunction with an amazing "image-processing computer" - the human brain!
Herschel's other dictum is that "When an object is once discovered by a superior power [a large telescope], an inferior one [a smaller telescope] will suffice to see it afterwards." The truth of this statement has been demonstrated countless times by visual observers from Sir William's time down to the present. One classic case involves the famed white-dwarf companion to the star Sirius. It took an 18-inch refractor to discover this minute object, but (depending on just where it is in its tight orbit about the primary) it can and has been seen in instruments as small as 4- to 6-inches in aperture!
The time spent studying and applying what follows will pay off significantly by enhancing your exploration and enjoyment of the wonders contained in the Herschel catalog. Readers desiring to delve even deeper into such matters -including the fundamentals of telescopes, eyepieces, mountings, and accessories, as well as their use - should consult the author's book A Buyer's and User's Guide to Astronomical Telescopes and Binoculars (Springer, 2007).
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