Dr Goring on Monochromatic Light, <S;c. 55 



enough. The blue ray, with an achromatic object-glass, per- 

 forms pretty well on the lined objects. I like it better than 

 any of the rest, (for it most resembles day-light,) though each 

 is capable of exhibiting the lines, if not procured in a state of 

 too great intensity, by being taken too close to the prism. 



I repeated these experiments with the flame of a wax taper 

 in a dark room in the following manner : — I used the prism as 

 before, and adjusted the wax light to a proper height to suit 

 it, I then carefully shaded the candle, so that none of its di- 

 rect light should get into the microscope, and threw the spec- 

 trum formed by the prism into the body of the instrument, 

 taking care to use it at such a distance from the prism that 

 the light of the taper should be fairly dispersed. I did not 

 use the long tube in these experiments, thinking it of no use, 

 as the room was dark, and I could not see my field of view 

 until the spectrum of the prism was directed into it. The 

 light afforded by the prism was too faint for the power I used 

 with the solar light. I was therefore obliged to reduce it one- 

 half. Even then the blue ray had not sufficient power ; the yel- 

 low one had, and seemed little different in its effect from the 

 taper itself in the same state of intensity. Suffice it to say, 

 that I got the same results as before, — that is, when I obtained 

 the light of the prism perfectly direct I obtained achromatism ; 

 when it came obliquely, the coloured fringes instantly appear- 

 ed, modified by the tint of the ray, and much in the same 

 way as if I had looked through a coloured eye-glass at the 

 image produced by common light. Wishing to sift this phe- 

 nomenon to the bottom, I then used the light of the taper in 

 its natural state, and found that there is one particular po- 

 sition in which it may be placed behind the stage, so as to 

 give apparent achromatism with any sort of microscope 

 whatever, however chromatic it may be. This appears to be 

 as nearly as possible in the axis of the microscope ; if it is 

 placed ever so little out of it, the coloured fringes instantly 

 show themselves. I am confident that I could make an un- 

 experienced person believe in the achromatism of any micro- 

 scope whatever by making this arrangement ;* and a most 



* Upon ;i careful re-examination, I <lo not think myself justified in spy- 

 ing that achromatism can really he so produced ; it should rather be termed 



