On Refraction. 253 
the erect image becomes considerably magnified, and the inverted 
images are seen forming a luminous circle around; and, as the 
lens is distanced, they contract or coalesce into one inverted image 
forming the focus. This may be esteemed an experimentum 
crucis. The following figure may serve to illustrate this inter- 
esting subject. & a lighted can- 
dle; a,a smallierect image formed 
on the convex surface of the lens 7 
by reflection,transmits the rays e, f, e +——\ 
to form the magnified image seen | ile 
by an eye at e, f. c,d, two in- a 
verted images formed by diverging “% 
rays striking and reflected from the ay 
concave surface, and travelling to 
g and A, forming a luminous circle, margined by orange rays. 
Dr. Herschel justly remarked that the greatest heat was beyond 
the focus or image; and I have found by repeated experiments 
that inflammation does not take place at the image of the sun, 
but on the crossing of the rays. The focus g, &, is produced by 
reflections from c, d, and therefore I would suggest the term re- 
flected instead of geometric focus. 
When we look at a book through a convex lens, the letters are 
not only magnified in all their dimensions, but they appear much 
blacker and better defined, and also much nearer to the eye. How 
could any bending of the rays produce these effects ? The inter- 
position of a semi-transparent substance, such as a glass lens, 
would undoubtedly diminish instead of increasing the brightness of 
the letters, if we took our ideas immediately from the object; but 
on the other hand, when we admit that an image is formed and 
painted on the posterior surface of the lens, this image being 
niuch nearer to the-eye would account for the appearance. 
I shall not enter into the subject of the identity of heat and 
light, further than to remark, that the heat is in all probability 
in a great measure to be attributed to the reflection of the rays 
from ¢ and d, and not from any separation of calorific and lumi- 
nous rays ; trdeadl the discovery already announced in your Jour- 
nal, and in the experimental outlines, that the prism has a ca- 
lorific focus, must for ever set at rest Dr.Herschel’s speculations ; 
and if I had no further argument in opposition to Sir Isaac New- 
ton’s opinions, this fact would be sufficient to convince any un- 
prejudiced reader that the solar ray was never separated by that 
great man. I am ready to admit that the calorific focus of a 
convex lens is somewhat removed from the focus of light. ‘The 
images at c,d, forma luminous circle, while the other rays re- 
flected at different angles, according to the augles of incidence, 
form 
