184« The Rev. J. B. Reade on 'producing Achromatic Light 



of my finest prisms this spectrum appears like a piece of 

 striped cloth. 



I 



XLI. Oil a Method of producing Achromatic Light in Solar 

 and Oxy-hydrogen Microscopes, and on the Effect of a Cur- 

 rent of Air upon the Rays that occasion Heat, By the Rev, 

 J. B. Reade, ill. y^.* 



T will no doubt be admitted, that the experiments of Mel- 

 loni on radiant heat and light have not only given us much 

 insight into the nature of these two agents, but have also 

 tended to solve the important problem which had been raised 

 as to their identity. Henceforward, therefore, any new facts 

 can occupy but a secondary place, and, however interesting 

 in themselves, their real value must depend on their conform- 

 ing to a theory which has already been independently proved. 



The received theory is, that the luminous and calorific rays 

 are two essentially distinct modifications which the aethereal 

 fluid suffers in its mode of existence, and that they may be 

 easily separated the one from the other by transmitting the 

 sethereal fluid through screens of different substances. 



Melloni, in his experiments, has employed a variety of ra- 

 diating sources, and received the rays on screens of coloured 

 and uncoloured glass, liquids, and crystallized bodies. The 

 tables with which he has furnished us of both solid and liquid 

 bodies, exhibit the common thickness of the screens employed, 

 and besides the substance, the indications of the thermomulti- 

 plier, and the number of rays transmitted as compared with 

 the whole radiation. The effect produced appears to vary 

 with nearly every variation of the substance, and it is only 

 with sulphate of copper, and a peculiar species of green glass 

 coloured by means of oxide of copper, that no calorific action 

 is perceptible. 



In following out Melloni's idea of the separation of the ca- 

 lorific and colorific raysf, I have sought in my own experi- 

 ments to attain this object by modes sufficiently effective in 

 themselves, and, at the same time, admitting of such easy ap- 

 plication to the solar and oxy-hydrogen microscopes, that 

 achromatic object-glasses, and objects mounted in balsam, may 

 be used without risk or danger. 



That method which I have found to be very successful, and 

 attended with the least possible amount of trouble, consists, as 

 I have stated in a paper communicated to the Royal Society, 

 in a certain position of the condensing lens and the field-glass 

 of the solar microscope, by which a difference of at least 50° 



* Communicated by the Author. 



[f A translation of Melloni' s paper on this subject will be found in 

 Scientific Memoirs, Part III. p. 388.— EniT.] 



