332 LECTURE XXXVI. 



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 denned, unless the ob- 

 jects 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 remote 

 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 ob- 

 liquely, 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 im- 

 pression 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 a speculum above it, which throws 

 the image through the lens, upon a flat surface placed below, on which the 

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

 impediment from the interception of the light by the hand and the instru- 

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

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

 lens, with a cover admitting the light only through a small aperture near 

 its centre, on the principle of Dr. Wollaston's periscopic spectacles,* an 

 image nearly flat might be obtained ; but in this case too much of the light 

 would be excluded. It has been usual to consider the image of a very dis- 

 tant object, formed by a convex lens, as a portion of a spherical surface of 

 which every part is equally distant from the centre of the lens ; but this 

 estimate is extremely erroneous, for the effect of the obliquity of the 

 different pencils of rays materially increases the curvature of the image. 

 In fact no pencil of rays, falling obliquely on a spherical surface, can be 

 collected any where to a perfect focus : the image of a circle would become 

 most distinct at one distance, and that of its diameter at another ; but for 

 both these images, the surface ought to be much more curved than that 

 which has been usually considered, and the mean of the curvatures re- 

 quired for them, which must be the best form for the ground or bottom of 

 a camera obscura, is equal to that of a sphere of which the radius is three 

 eighths of the focal distance, when a double convex lens of crown glass is 

 employed. (Plate XXVIII. Fig. 397. . .399.) 



In the solar microscope, an image is formed on a wall or screen, by 

 means of a lens of small focal length, near to which the object is placed, so 

 that the image is very much magnified. For this purpose the room must 

 be darkened, and the object strongly illuminated by the sun's light, which 

 is condensed by means of a large lens, and sometimes by two or more lenses 

 placed at a distance from each other ; but care must be taken to avoid 

 burning the object by bringing it exactly into the focus ; and, on the other 

 hand, if it be much beyond the focus, the light will be thrown upon a small 

 part of the image only ; the best arrangement appears to be, to bring the* 

 focus of the condensing lenses very near to the small lens ; and in order to 

 * Nicli. Jour. vii. 143, 241. 



