290 Mr. Powell oi Solar Light and Heal. [Oct. 



length : this was compared with a simple lens of about the 

 same focal length, having its aperture diminished by a diaphragm 

 to the same size, and at its central part about the same thick- 

 ness as the compound one. The results of this comparison are 

 given in Exper. 13 and 14; and it is there, I conceive, quite 

 clear, that the exterior effect is as nearlu as possible equal in both 

 cases. Hence, I think, I am warranted in concluding, to what- 

 ever sort of rays the phenomena is owing, the principal part of 

 them at least are not of the same kind, or subject to the same 

 affections and modifications, as those refracted by the lenses. 



(52.) Having by me a smaller instrument of the same kind, I 

 thought it worth while, for the sake of comparison, to make a 

 few observations with it. This instrument is of the " portable " 

 description, having its plain bulb in a line under the other. The 

 apparently less indications in Exp. 15 and 16 arise only from the 

 two instruments not being adjusted to the same point as the 

 zero. In these results, as I before observed, the action on the 

 plain bulb is, perhaps, more conspicuous than in any former 

 instances. 



(63.) I was extremely desirous of trying the relations of this 

 heating power to transparency in screens. This, in some mea- 

 sure, might have been ascertained by means of the small instru- 

 ment when in its glass case as compared with the effect when 

 the case was removed ; but I have not given any of these results, 

 as I found it extremely difficult to pass the rays into the small 

 cylindrical case of this instrument without producing reflections 

 from its inner surface which totally interfered with the results. 

 In some instances, as in No. 16 compared with No. 15, the effect 

 on the blackened bulb was as great without the case as with it. 

 This seems to be in favour of the non-transmissibility of the 

 effect through glass, because, in the absence of the case, it 

 ought to have been very considerably less. To attempt the 

 application of plates of giass as screens in cases of so delicate a 

 nature, and when the screen must almost touch the bulb, 

 appeared quite impracticable on account of the cooling effect of 

 the glass. In this respect I must be content to leave the inves- 

 tigation imperfect. It is, however, most probable, that with 

 large and powerful lenses the effect might be sufficiently great 

 to admit of some application of this kind. As well in respect 

 to this point as to others, I am willing to confess the imperfec- 

 tions of the present inquiry. I take this opportunity, therefore, 

 of saying, that I should feel the greatest interest in hearing that 

 a similar train of investigation had been carried on by those who 

 may be possessed of large lenses, or may be able to devise more 

 accurate and satisfactory methods of operating. 



(64.) From this series of experiments, in its present state I 

 shall not attempt to draw any general or theoretical inferences, 

 but shall content myself with remarking, that so far as these 



