Dr Goring- on Monochromatic Light, ^c. 53 



It is well known to practical men, that the most vivid white 

 light which can be procured, barring always that of the sun, 

 is by no means too brilliant for opaque objects ; it can need, 

 therefore, no argument to show, that, at all events, monochro- 

 matic light can never answer for these, however much it may 

 be condensed ; for transparent ones with low powers and large 

 apertures it might, were it not for some circumstances to be 

 hereafter stated. Monochromatic light must of course always 

 be of some colour or another, which circumstance, of course, 

 must produce a very fallacious appearance in the vision of most 

 objects, which will compel the observer to examine them afresh 

 with white light, in order to rectify their apparent tints. More- 

 over, I myself am convinced that a real monochromatic light 

 is yet to seek, and shall here state the experiments on which I 

 found my opinion, leaving it to others better acquainted with 

 the subject than myself to ascertain whether I am in error or 

 not. When white light is subjected to the action of a prism, 

 it is well known that it is dispersed or resolved into seven pris- 

 matic rays, leaving a number of colourless lines, when the 

 experiment is made with due delicacy in the method prescrib- 

 ed by Fraunhofer. Now, if there is any such thing as mono- 

 chromatic light, assuredly a prism ought to be able to produce 

 it ; and, accordingly, I have chosen light so generated for my 

 experiments. 



The white light of the sun reflected by a plane mirror is 

 able to show most of the coarser proof-lined objects in a rough 

 manner ; indeed, I have sometimes succeeded in bringing out 

 the straight lines on the Podura with it, and this too with an 

 instrument which had not the power of showing them with any 

 other species of illumination. I therefore chose this class of 

 objects in my trials with solar light, for they are, upon the 

 whole, exhibited better in proportion by it than any other bo- 

 dies. In order, however, to obtain from the solar beam a 

 monochromatic illumination, I made the following arrange- 

 ments : — I placed a compound microscope with its body in a 

 horizontal position, and excluded all while extraneous light 

 from it as follows : — I attached a tube six inches long, to the 

 stage, and at the end of it fixed a diaphragm J of an inch in 

 diameter in the prolongation of the axis of this tube ; and, al 



