Then place a Lens of about three foot radius (suppose a broad Object-glass of a three foot Telescope), at the distance of about four or five foot from thence, through which all those colours may at once be transmitted, and made by its Refraction to convene at a further distance of about ten or twelve feet.
The triple object-glass, consisting of a combination of two convex lenses of crown glass with a concave flint lens between them, was introduced in 1765 by Peter, son of John Dollond, and many excellent telescopes of this kind were made by him.
If then the objective tube is directed to any star, the convergent beam from the object-glass is received by the plane mirror from which it is reflected upwards along the polar axis and viewed through the hollow upper pivot.
Telescope with a concave eye-lens instead of an eye-piece still survives as the modern opera-glass, on account of its shorter length,_ but the object-glass and eye-lens are achromatic combinations.
It may be fixed at the end of a tube, of a suitable length to its focal distance, as an object-glass, - the other end of the tube having an eye-glass fitted as usual in astronomical telescopes.
Pp. 199-209) the rays from the object-glass pass successively through lenses as follows: The lens b is divided, and one of the segments is moved by a micrometer screw.
If we desire to, utilize all the parallel rays which fall upon an object-glass it is necessary that the full pencil of emerging rays should enter the observer's eye.
These changes may and do arise from the following causes: (i.) The focal length of the object-glass and the length of the tube are affected by temperature.
" Thus the magnitude of the central spot is diminished, and the brightness of the rings increased, by covering the central parts of the object-glass."
It must be remembered, however, that when Dollond expressed preference for this third type he had not then invented the achromatic object-glass.