58 THE ACT OF SIGHT. 



picture formed on the retina of the eye, that gives 

 us notions of distance and also of magnitude. The 

 formation of that picture, though, as there is little 

 doubt it does, it takes place in the living eye, is, 

 after all, merely a mechanical matter ; and any one 

 can produce it at pleasure, by closing the shutters, 

 boring a hole in them, placing a glass which is con- 

 vex, or thick in the middle, in the opening, and hold- 

 ing a sheet of paper at the proper distance behind. 

 Not only that, but, by means of mirrors properly 

 placed, or prisms of glass, which reflect from their 

 hinder surfaces, we can convey those images of visi- 

 ble things whithersoever we will. That beautiful 

 contrivance of Ramsden's, which, from being in itself 

 invisible, is called " Ramsden's ghost," is a remark- 

 able instance of that. In a fine astronomical instru- 

 ment for taking the elevations of the celestial bodies, 

 it is necessary that, the plummet should, by means 

 of the spider's thread, or whatever other delicate 

 substance is used for marking it, pass in a down- 

 ward line, from the very centre of the axis on which 

 the instrument turns to the very centre of the earth. 

 The axis itself is enclosed in the workmanship, so 

 that the observer cannot see it, or make any direct 

 reference to it for adjusting his instrument; but 

 Ramsden's ghost brings it faithfully to his view, let 

 the path be ever so intricate or circuitous. On the 

 axle there is a dot no bigger than a pin's point : one 

 prism receives the light from that, reflects it to an- 

 other, that to a third, and so on, till the picture of it is 

 thrown upon the limb of the instrument, just where 

 it is crossed by the spider's thread of the plummet ; 

 and as those prisms are all perfectly parallel, the 

 reflection is made to fall on the limb more exactly 

 under the very centre of the axis than any one could 

 discover by immediate observation. Thus we can, 

 by means that are perfectly mechanical, do even 

 more than eyes can do in the forming of a picture on 

 the retina. Therefore, we are warranted in coiv 



