646 



EXPERIMENTS AND CALCULATIONS 



upon considering the phenomena of theaber- 

 ation of the stars, I am disposed to believe, 

 tliat the luminiferous ether pervades the sub- 

 startce of all material bodies with little or no 

 resistance, as freely perhaps as the wind 

 l)asses through a grove of trees. 



The observations on the effects of diffrac- 

 tion and interference may perhaps sometimes 

 be applied to a practical purpose, in making 

 us cautious in our conclusions respecting the 

 appearances of minute bodies viewed in a 

 microscope. The shadow of a fibre, how- 

 ever opaque, placed in a pencil of light, ad- 

 mitted through a small aperture, is always 

 somewhat less dark in the middle of its 

 breadth than in the parts on each side. A 

 similar effect may also take place, in some 

 degree, with respect to the image on the re- 

 tina, and impress the sense with an idea 

 of a transparency which has no real exist- 

 ence: and, if a small portion of light be 

 really transmitted through the substance, this 

 may again be destroyed by its interference 

 with the diffracted light, and produce an ap- 

 pearance of partial o|)acity, instead of uniform 

 semitransparency. Thus, a central dark spot, 

 nnd a light spot, surrounded by a darker circle, 

 may respectively be produced in the imagesof 

 a semitransparent and an opaque corpuscle, 

 and impress us with an idea of a complication 

 of structure which does not exist. In order to 

 detect the fallacy, we may make two or three 

 fibres cross each other, and view a number 

 of globules contiguous to each other ; or we 

 may obtain a still more effectual remedy, by 

 changing the magnifying power; and then, 

 if the appearance remain constant in kind 

 and in degree, we may be assured that it 

 truly represents the nature of the substance 

 to be examined. It is natural to inquire 



whether or no the figures of the globules of 

 blood, delineated by Mr. Hewson in the Phi- 

 losophical Transactions (LXIII, for 1773,) 

 might not in some measure have been influ- 

 enced by a deception of this kind. As far 

 as I have hitherto been able to examine the 

 globules, with a lens of one fiftieth of an inch 

 focus, I have found them nearly such as Mr. 

 Hewson has described them : Mr Cavallo 

 has, however, published, in his essay on fac- 

 titious airs, some observations which strongly 

 confirm the suspicion of an optical fallacy, 

 and which agree precisely whh the theory 

 here advanced. 



V. REMARKS ON THE COLOURS OF NATURAL 

 BODIES. 



Exper. 5. I have already adduced, in illus- 

 tration of Newton's comparison of the colours 

 of natuifil bodies with those of thin plates, Dr. 

 Wollaston's observations on the blue light 

 of the lower part of a candle, which appears, 

 when viewed through a prism, to be divided 

 into five portions. I have lately observed a 

 similar instance, still more strongly marked, 

 in the light transmitted by the blue glass sold 

 by the opticians. This light is separated by 

 the prism into seven distinct portions, nearly 

 equal in magnitude, but somewhat broader, 

 and less accurately defined, towards the vio- 

 let end of the spectrum. The first two are red, 

 the third is yellowish green, the fourth green, 

 the fifth blue, the sixth bluish violet, and the 

 seventh violet. This division agrees very nearly 

 with that of the light reflected by a plate of 

 air s-^-fo of an inch in thickness, correspond- 

 ing to the 1 1th series of red, and the 18th of 

 violet. A similar plate of a metallic oxid 

 would perhaps be about t-totto of an inch in 

 thickness. But it must be confessed, that 



