It can be made to suit any latitude, and by the addition of foot screws on a level floor, can be got into suitable adjustments, and if it has to be removed (for the stand is very little heavier than an ordinary Alt-Azimuth), “guides” can be provided, so that it can go very approximately into the exact place again when brought out for use. They are made, when desired, with a revolving body and fine screw-motion in declination.
The speculum is 5-1/4 inch diameter, and carefully figured, and is recommended as a very useful instrument. The size and power of this telescope has been adopted as that most likely to meet the means and requirements of a large number of amateurs. Many prefer to commence astronomical observations with “_something inexpensive_,” and are led to begin with the popular and well advertised 3 inch refractor at £5. These, except in a few chance cases, are sure to prove disappointing, and are perhaps the cause of their giving up any further attempts to follow up the subject of astronomy, which may otherwise be so pleasantly and profitably pursued with a reliable instrument.
It is well known that a less aperture than 4-1/2 inches is insufficient to give the observer a satisfactory idea of the varied and most interesting details of the planets, and the ever-changing outline and tone of the belts and markings of Jupiter, Saturn, and Mars. Large apertures bring out _details_ when the smaller ones can only show a _general outline_. The former also delineates more with a lower power, providing the focus is of proper length.
The planets and nebulæ cannot be seen to advantage without aperture and focal length. The field of view is then flat, the object is properly illuminated, with sharp and crisp definition, and is also _much_ less subject to annoyance from tremour, through the necessity for constant adjustments in the fields, &c., as is the case with small apertures, for the object is magnified more in proportion. A certain magnifying power is necessary in order that the object may be sufficiently large to scrutinize; and this, whether the aperture is large or small, must be from about 150 to about 300 times for the planets. But 150 on a 3 inch is a high power for the quantity of light obtained, to say nothing of the separate consideration and advantage of long focus. Aperture is a quality or function of the telescope considered separately from light or focus. For instance, suppose a 5 inch speculum is so thinly silvered that it gives the exact degree of light as a 2-1/2 inch refractor, the defining and separating power of the 5 inch aperture would be very superior to the 2-1/2 inch.