37 6 PHYSIOLOGICAL PHYSICS. ichap. xxix. 



shortened at pleasure. The focussing for different 

 distances is also effected by altering the position of the 

 lens, in reference to its distance from the plate, and 

 this is clone by the screw r. Finally we have 

 noted (page 348) that spherical aberration interferes 

 with the distinctness of images, and that this is got rid 

 of by cutting off outside rays proceeding from the 

 object. In the camera this is accomplished by insert- 

 ing a diaphragm, through a slit in the lens tube, 

 between the glasses of the lens. In the diaphragm a 

 central hole is pierced, a diaphragm with a large or 

 small hole being used according as the light is feeble 

 or strong. 



Now the EYE is to be regarded as a camera obscura 

 with a small hole in front, through which rays of light 

 pass, and with refractive media. The sclerotic and 

 choroid coats form the walls ; the cornea, aqueous 

 humour, crystalline lens, and vitreous body are 

 different refractive media, but they all tend to effect 

 the same purpose, to bring parallel rays of light to a 

 focus on the sensitive coat, the retina, and so to form 

 there a sharp, real, and inverted image of the object. 

 There is also a focussing arrangement for always 

 bringing the clear image on to the retina, in spite of 

 varying distances of the object. Lastly, the iris with 

 its pupil acts as a diaphragm, contracting with strong 

 light so as to limit the rays, and dilating with little 

 light so that more rays pass through. 



In the eye the converging apparatus does not 

 consist of a single refracting medium. There is the 

 aqueous humour, separated from air by the convex 

 cornea, and the aqueous and vitreous humours, separ- 

 ated from one another by the more dense crystalline 

 lens. The refraction effected by the cornea alone 

 would bring rays, falling on the eye from a distance, 

 to a focus about 10 millimetres behind the retina, and 

 it is the additional convergence produced by the 



