ox OPTICAL insthumexts. 425 



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 distant 

 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 ex- 

 tremely erroneous, for the eflectof the obrujuity of the different pencils of rayg 

 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 required 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 XXV^II. Fig, 

 397 . . 399. ) 



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

 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 burn 

 ing 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 adjust 

 the instrument in the most convenient manner, the distances of all the 

 lenses ought to be moveable at pleasure: the want of this precaution is a 

 material defect in the usual construction of the instrument. The speculum 

 which first receives the light must be capable of motion in all angular direc- 

 tions, in order to allow us to accommodate its position to the changeable 

 place of the sun; and the adjustment has sometimes been performed by 

 means of a heliostate, an instrument calculated for turning the speculuaj 



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