834 LECTURE XXXVI. 



an inverted image to be formed between them, and to throw a second 

 picture of this image on the screen, in its natural erect position, unless the 

 object be of such a nature that it can be inverted without inconvenience. 

 This effect was very well exhibited at Paris by Robertson ; he also com- 

 bined with his pictures the shadows of living objects, which imitate toler- 

 ably well the appearance of such objects in a dark night, or by moonshine : 

 and while the room was in complete darkness, concealed screens were pro- 

 bably let down in various parts of it, on which some of the images were 

 projected ; for they were sometimes actually situated over the heads of the 

 audience. (Plate XXVIII. Fig. 401.) r- 



In almost all telescopes and compound microscopes, the image formed 

 by one lens or mirror stands in the place of a new object for another. 

 The operation of such instruments may be illustrated by placing a screen 

 of fine gauze at the place of the image, which receives enough light to 

 make the image visible in all directions, and yet transmits enough to form 

 the subsequent image. The simplest of such instruments is the astrono- 

 mical telescope. Here the object glass first forms an actual inverted image, 

 nearly in the principal focus of the eye glass, through which this image is 

 viewed as by a simple microscope, and therefore still remains apparently 

 inverted. In order to find the angular magnifying power, we must divide 

 the focal length of the object glass by that of the eye glass : this quotient 

 is consequently the greater as the focal length of the object glass is greater, 

 and as that of the eye glass is smaller : but the power of the instrument 

 cannot be increased at pleasure by lessening the focal length of the eye 

 glass, because the object glass would not furnish light enough to render the 

 view distinct, if the magnifying power were too great. (Plate XXVIII. 

 Fig. 402.) 



The double or compound microscope resembles in its construction the 

 astronomical 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, 

 which 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 

 advantage 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 



