166 Intelligence and Miscellaneous Articles, 



us to be implied in the passage that we quoted that this theory ex- 

 tended to the explanation of the light and heat of combustion only. 

 We do not think it needful to add anything in defence or expla- 

 nation of our other criticisms, since Dr. Apjohn himself admits their 

 justice ; but with regard to his statement that he had detected 

 several of the faults which we have pointed out, and that they will 

 be corrected in a new edition, we may remind him that, had this 

 statement accompanied the copy of his book sent for review, we 

 should probably have contented ourselves with laying it before 

 our readers and recommending any who intended purchasing the 

 work to wait for the second edition. And since we now understand 

 that a new edition is likely soon to appear, we may further remind 

 him that his having had on his list of errata all the errors, " with a 

 single exception," pointed out in our review, is no proof of the com- 

 pleteness of his revision, for we made no attempt to enumerate all 

 the faults we had detected : our quotations were, as we have already 

 said, intended merely as specimens to prove that our criticisms were 

 not made without good reason ; and in case of need we are quite pre- 

 pared to produce at least as many more of very similar quality. 



ON THE MEASUREMENT OF THE CHEMICAL BRIGHTNESS OF 

 VARTOUS PORTIONS OF THE STINTS DISK. BY THOMAS 



WOODS, M.D. 



Professor Roscoe's paper " On the Measurement of various por- 

 tions of the Sun's Disk," read to the Royal Society, an abstract of 

 which appears in the May Number of this Magazine, is extremely 

 interesting. The chemically active rays decrease in intensity from 

 the centre to the circumference, which the Professor found by expo- 

 sing a prepared paper in a camera to the action of the sun's picture, 

 and comparing the shade of tint produced thereby at the centre and 

 at the circumference with a certain standard. I would, however, 

 suggest the plan I described in this Magazine in July 1854. It 

 consists in exposing the prepared paper to the sun's picture in the 

 camera for a period so short that the centre or most active rays only 

 have time to act on it ; then for the next impression to leave the 

 paper exposed for a somewhat longer time, so that a somewhat larger 

 picture is obtained ; and so on until the entire picture is given. For 

 instance, suppose the sun's picture is divided into zones by concen- 

 tric circles thus, and suppose the centre rays could 

 affect the prepared paper in one second, the second 

 zone in two seconds, the third in three seconds, and 

 the circumference in four ; then by exposing the 

 paper for these periods of time a corresponding 

 amount of the disk would be obtained ; the size of 

 the impression produced would be in proportion to the time of expo- 

 sure ; and the intensity of the rays from any part of the disk would 

 be more accurately fixed by once getting the time required for their 

 action, and more permanently, I fancy, than by the use of the stand- 

 ard tints. This was the plan I adopted in 1854 to show the identity 



