FIXED LINES IN THE SPECTRUM. 47 



ed by a lens and dispersed by the action of a prism, is as is given in Jig. 118; where 

 the triangles overlap at the bases, A B, and the intermixture of the different coloured 

 rays is at a maximum, but at the other side of the spectrum the vertex of each triangle 

 being separate from those on either side, the colours are, of course, in a state of purity. 



160. These are the chief improvements which were introduced by NEWTON in the 

 formation of the solar spectrum, and it was in one purified in the manner here descri- 

 bed that his most delicate experiments were made. It is extraordinary, that after he had 

 advanced so far as to the use of a lens, and a narrow longitudinal aperture, that the 

 discovery of the fixed lines escaped him. Even if his glass prism was imperfect, these 

 lines are easily seen with prisms made from pieces of looking-glass cemented together 

 and filled with a solution of sugar of lead, and such prisms NEWTON says that he em- 

 ployed. 



161. The dark lines in the solar spectrum were first perceived by Dr. WOLLASTON, 

 and described by FRAUNHOFER. They may be seen by forming on a screen of white 

 card-board, jig. 2, Frontispiece, the spectrum of a narrow fissure or chink. The lens 

 may be a common convex lens, or an achromatic ; the prism should be without veins 

 or other flaws ; under these circumstances, as soon as the screen is brought into the fo- 

 cal position, so that the spectrum is produced with sharp edges, a number of dark lines 

 will be seen crossing it in certain positions. Of these, a sketch is given in Jig. 103, and 

 in the frontispiece. These different lines have been designated by FRAUNHOFER by the 

 letters of the alphabet, as follows : A is in the red region, B in the red near its outer 

 end, C is beyond the middle of the red, D is in the orange, and is a double line, E is 

 in the green, F in the blue, G in the indigo, and H in the violet. 



162. Of the fixed lines, it is only the larger which can be seen by projection upon a 

 screen. Viewed through a telescope, a vast number of minute ones become visible. 

 FRAUNHOFER estimated them at GOO, and these are now known to constitute a part only 

 of those which actually exist. In the solar spectrum, they maintain their position 

 among the different coloured spaces, and the same observation applies to the light which 

 comes by reflection from any of the planetary bodies. In artificial lights they are not 

 seen ; and in the rays which come from the various fixed stars, although they are pres- 

 ent, they are found in different positions and in different groups. 



163. It has already been stated that when any salt of silver is exposed to the spec- 

 trum (149), a decomposing effect takes place, and the salt becomes black. If the spec- 

 trum used be one of a narrow fissure, so that the fixed lines are visible, it will be found 

 that the resulting impression contains them ; that where there is a fixed line there is a 

 corresponding want of action upon the decomposed surface ; and, farther, that beyond 

 the extreme violet end, and in a region where the eye can perceive no light, the chem- 

 ical impression reveals remarkable groups of these inactive spaces, M N O ~P,Jig. 103. 

 These extra-spectral lines were discovered about the same time by M. BECQUEREL in 

 France, and by the author of this book. M. BECQUEREL'S description of them was, 

 however, published first. Their use has become essential in all chemical investigations 

 connected with light. 



, 164. Without anticipating here what will have presently to be rtated in regard to 



