428 LECTURE XXXVI. 



tronomical telescope, except that the distance of the lenses much exceeds their 

 joint focal length; and the angular magnitude is greater than when the 

 same object is viewed through the eye glass alone, in proportion as the first 

 image is further from the object glass than the object itself. (Plate XXVIII. 

 Fig. 403.) 



In the Galilean telescope, or opera glass, a concave eye glass is' placed so 

 near the object glass, that the first image would be formed beyond it, and 

 near its principal focus; and the second image, formed by the eye glass, 

 Avhich is the virtual image viewed by the eye, being on the opposite side of the 

 centre, is inverted with respect to the first image, and erect with respect to 

 the object. In this case also the magnifying power is indicated by the 

 quotient of the numbers expressing the focal lengths of the glasses. (Plate 

 XXVIII. Fig. 404.) 



The inverted image of the astronomical telescope may be made erect by 

 means of an additional eye glass. In the common day telescope of Rheita, 

 two such eye glasses are employed, of nearly equal focus, which have the ad- 

 vantage of procuring a greater extent in the field of view; they are usually 

 so placed as to have little or no effect on the magnifying power. (Plate 

 XXVIII. Fig. 405.) 



Dr. Herschel's reflecting telescopes resemble, in their effects, the simple 

 astronomical telescope; a concave speculum, or mirror, being substituted 

 for the object glass, and the eye glass being so placed as to magnify the 

 image formed by the speculum. But smce the speculum, if it received the 

 principal rays perpendicularly, would send them back in the same direction, 

 it is necessary, in this construction, to have them reflected somewhat ob- 

 licjuely, the speculum being a little inclined to the axis of the telescope, in 

 order that the light may have free access to it. An arrangement of this kind 

 Avas proposed long ago by Maire, but it has been very little employed before 

 Dr. Herschel's time. This excellent philosopher and mechanic has carried 

 the perfection of his telescopes to a degree far exceeding all that could have 

 been expected from the labours of former opticians. His instruments allow 

 liim to extend the linear dimensions of his objects several thousand times : 



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