ON OPTICAL INSTRUMENTS. 427 



mg a strong light upon an actual opaque object, or on a living person, its 

 image is formed on the curtain, retaining its natural motions : but in this 

 case the object must be considerably distant, otherwise the images of its 

 nearer and remoter parts will never be sufticiently distinct at once, there- 

 fraction being either too great for the remoter, or too small for the nearer 

 parts: and there must also be a second lens, placed at a sufficient distance 

 from the' first to allow an inverted image to be formed between them, and 

 to throw a second picture of tliis image on the screen, in its natural erect 

 position, unless the object be of such a nature that it can be inverted with- 

 out inconvenience. This effect was very well exhibited at Paris by Robert- 

 son; he also combined with his pictures the shadows of living objects, 

 which imitate tolerably well tlie appearance of such objects in a dark night, or 

 by moonshine: and while the room was in complete darkness, concealed 

 screens were probably 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.) 



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 sub- 

 sequent image. The simplest of such instruments is the astronomical tele- 

 scope. 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 (juotient is conse- 

 quently 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 as- 



