30 jVatural theology. 



construct optical instruments. We find them per- 

 fect for the purpose of forming an image by re- 

 fraction : composed of parts executing ditTerent 

 offices: one part having fulfilled its office upon the 

 pencil of light, delivering it over to the action of 

 another part : that to a third, and so onward : the 

 progressive action depending for its success upon 

 the nicest and minutest adjustment of the parts 

 concerned : yet these parts so in fact adjusted as 

 to produce, not by a simple action or eftect, but by 

 a combination of actions and effects, the result 

 which is uhimately wanted. And forasmuch as 

 this organ w'ould have to operate under different 

 circumstances, with strong degrees of light and 

 with weak degrees, upon near objects and upon 

 remote ones, and these differences demanded, ac- 

 cording to the laws by which the transmission of 

 light is regulated, a corresponding diversity of 

 structure, — that the aperture, for example, through 

 which the light passes should be larger or less — 

 the lenses rounder or flatter — or that their dis- 

 tance from the tablet upon w^hich the picture is 

 delineated should be shortened or lengthened, — 

 this, I say, being the case, and the difficulty to which 

 the eye w^as to be adapted, we find its several 

 parts, capable of being occasionally changed, and 

 a most artificial apparatus provided to produce 

 that change. This is far beyond the common 

 regulator of a watch, which requires the touch of 

 a foreign hand to set it ; but it is not altogether 

 unlike Harrison's contrivance for making a watch 



