40 THE ACT OF SIGHT. 



downward line, from the very centre of the axis on 

 which the instrument turns, to the very centre of 

 the earth. The axis itself is inclosed in the work- 

 manship, so that the observer cannot see it, or make 

 any direct reference to it for adjusting his instru- 

 ment ; but Ramsden's ghost brings it faithfully to 

 his view, let the path be ever so intricate or cir- 

 cuitous. On the axle, there is a dot no bigger than 

 a pin's point : one prism receives the light from that, 

 reflects it to another, 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 imme- 

 diate 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 concluding, that 

 that is not the act of sight, but that there is some- 

 thing mental consequent upon it, far more nice 

 and curious than any thing which material eyes 

 can discriminate. And we have proof of it, in 

 those pictured scenes which, sleeping or waking, 

 arise to the imagination, far different from any 

 thing that the eyes ever beheld, and yet equally 

 bright and perfect in the colouring. But those 

 imagined views are, in truth, all made up of that 

 which has been seen, or otherwise perceived by the 

 senses ; and, therefore, though, after observation 

 has given us the materials, we can, by the opera- 

 tion of our minds, work it into endless forms and 

 combinations of delight, we must obtain the mate- 

 rials originally from observation. Nor must we 



