424 LECTURE XXXVI. 



are more .expensive, and are liable to tarnish. Where a large mirror is re- 

 quired, with a weak reflection only, we may employ a single surface of 

 glass, the back of the piece being covered with a black coating of some 

 substance diftering little from glass in its refractive density, by means of 

 which the second reflection is avoided. 



When the image formed by a lens or mirror is received on a smooth but 

 unpolished surface, which is capable of irregular reflection, it is visible in 

 every direction. Such an image is exhibited in the camera obscura, the 

 solar microscope, and the magic lantern, or lucernal microscope. 



The general effect of the camera obscura is the same as may often be 

 observed in a dark room, where there is a small hole in the window shutter- 

 the great masses of light and shade, before the windows, being represented 

 an an inverted position, in the parts of the room diametrically opposite to 

 them, which are illuminated in dift^erent degrees, according to the quantity 

 of light which can reach them in straight lines from the external objects. 

 A lens, of a focal length somewhat smaller than the distance of the surface 

 on which the picture is projected, renders the images much more distinct; 

 but some of them are unavoidably imperfect and ill defined, unless the 

 objects happen to be situated at the same distance from the aperture ; for 

 the focus of the lens can never be adjusted at once to nearer and more re- 

 mote objects; nor would the picture be rendered more natural by such an 

 adjustment, for it would present to the eye at one view, with equal distinct- 

 ness, objects which never can be seen at once without some degree of con- 

 fusion. Sometimes the picture is intercepted, by a speculum placed obliquely, 

 and is thrown upwards on the surface of a plate of ground glass, upon which 

 its outline may be traced with a black lead pencil, and an impression may 

 be taken from it on moist paper, which will represent the natural situation 

 of the objects without inversion. Another arrangement is, to place the 

 lens horizontally, with the speculum above it, which throws the image 

 through the lens, upon a flat surfiice placed below, on which the objects 

 may be delineated in their natural position, but not without some impedi- 

 ment from the interception of the light by the hand and the instrument 

 employed. Such a surface, however, ought not to be perfectly flat, in 

 order to aiford the most distinct image, although by means of a meniscus 



