32 On the Halo seen around all Bodies. 



45. The lenticular character of the halo is indisputable, and 

 every experiment will the more fully establish the fact. It is a 

 true reflecting and magnifying medium^ and it is entirely owing 

 to this circutnstance that a convex le?is of glass possesses its mag- 

 nifying power. The mere suhstaitce or material of glass is only 

 necessaiy or accessory to the production of lenticular phenomena 

 in consequence of its capability of curvature and extension. 



46. Therefore, beyond a certain point a halo cannot maintain 

 its continuity, unless it have a solid or fluid medium on which it 

 can expand and keep its particles in contact. Nor can its ulti- 

 mate magnifying power be developed unless the material in which 

 it rests is convex, for being convex itself it requires a continued 

 extension of convex surface if greater magnifying power is re- 

 quired. An aperture which is of twice the diameter of the halo 

 will have an open space in the center free from it. This center, 

 therefore, is no lens, but if we put a convex glass in the aperture, 

 the halo then has a conducting medium, and can spread itself, or 

 rather connect itself with the halo on the glass, and thus exhibit 

 all the powers of a lens. 



47. It is the continuity of the halo throughout all the space of 

 a small circular aperture which gives the aperture, or the convex 

 glass within it, the character of a lens. In consequence of this 

 continuity, lenses can be built up of many pieces, and of course 

 there is no limit to their diameter. Convexity beitig the sole re-' 

 quisitefor a magnifyitig poiver, it is immaterial whether the lens 

 be of solid glass, or whether the two convex surfaces be of the 

 thinnest glass, cemented at the circumference and the hollow 

 space filled with a fluid. If the lens be built up of many pieces, 

 the blocks should all run parallel with the axis of the glass and 

 the central block should be of one piece throughout its axis, so 

 that there may be no interruption of the rays of light through it. 

 It is of no consequence how narrow the diameter of the block 

 may be, for the rays which are to give impressions of external 

 objects converge to a minute point on the apex of this block and 

 pass in a straight line to the apex of the axis of our own eye : the 

 present theory among philosophers is that the rays cross each 

 other in the cetitre of the lens, but this is an error which will 

 soon be corrected. 



