324 



THE SENSORIAL FUNCTIONS. 



a double convex lens; anil it has the property of collecting 

 into a focus rays proccedinj; from distant points."^ 



410 



Ilavinii: obtained this instniincMil, wc mav now venture 

 to enlarge the a])ertui'e tlii-oni;h wlilch the light was admit- 

 ted into our dark chamber, and fit into the apertivrc a dou- 

 ble convex lens. Vv'e hnve thus constructed the well known 

 optical instrument called the Camera Ohscura, in which 

 the imaj];es of external objects are formed upon a white sur- 

 face of paper, or a 5cnii-transj)arent ])latc of glass; and these 

 images must evidently bo in an inverted position with re- 

 •pect to the actual objects which they represent. 



Such is precisely the construction of the eye, which is, to 



• The refraction by spherical surfiiccs docs not, strictly speakint^, unite x 

 pencil of pnrallel or divergent rays into a muthematicul point, or focus; for 

 in reality the rays wiiich are near the central line are made to converg-e to a 

 point a little more distant than that to which tiie remoter rays converg-e: an 

 •ffect which 1 have endeavoured to ilUistrate by the diagram Fig-. 411} where, 



in order to render it obvious to tlie eye, the disparity is much exaj^fgerated. 

 But, on ordiuMiy occiisions, where great nicety is not required, this differ- 

 ence in the degree of convcrgiMicc l)et\vcen the central rays and tliose near 

 the circumference of titc lens, giving r'.sc to \vi;at is termed the Aberration 

 ef Sphericity, is too small to attract notice. 



