ON INSTRUMENTS AND THEIR USE. 15 



composed of lead and tin. Upon asking them what 

 substance they were handling they might probably 

 guess, perhaps not ; they would poise it in their 

 hands, look at it, smell it, try to cut it, perhaps, 

 examine its metallic lustre, and it would be very 

 odd indeed if they could agree as to the composition 

 of the alloy, unless settled by an assay upon the 

 balance. 



Has it ever occurred to the reader that such pro- 

 cesses as these go on in Microscopy, and that it is 

 necessary to carefully study the organ of vision in 

 order to gain a true insight into the object presented 

 to us ? On reference to plate 1, it will be seen that 

 the eye is a nearly spherical ball, capable of many 

 movements in its socket. It possesses an outer 

 translucent covering called the sclerotic coat (or 

 simply sclerotica) which may be seen at S. This is 

 thick, horny, and opaque, except in its anterior 

 portion. 



This sclerotic coat envelopes about f of the eye- 

 ball, and in common parlance is called the white of 

 the eye. 



The anterior transparent portion is called the 

 cornea, and has the shape of a very convex watch 

 glass. It is through this membrane that the light 

 passes to the interior of the eye. The cornea and 

 the interior portion of the sclerotica are covered 

 with a mucous membrane. 



Behind the cornea is a diaphragm of annular form 

 called the iris ; it is coloured and opaque, the 

 circular aperture in its centre, p, being called the 

 pupil. 



The iris, i 9 serves the purpose of regulating the 

 admission of light ; it varies in colour in different 



