PHYSICAL OPTICS, J39 



parency. Thus, a central dark fpot, and a light fpot far- 

 |-ounded by a darker circle, may refpe6tively be produced 

 in the images of a femitranfparent and an opaque corpufcle ; 

 and imprefs us with an idea of a complication of firu61ure 

 which does not exift. Jn order to deted the fallacy, we 

 may make two or three fibres crofs each other, and view a 

 pumber of globules contiguous to each other j or we may 

 obtain a flill more effectual remedy by changing the magni- 

 fying power ; and then, if the appearance remain conftant 

 in kind and in degree, we maybe atfured that it truly repre- 

 fents the nature of the fubftancc to be examined. It is na- 

 tural to inquire whether or no the figures of the globules of 

 blood, delineated by Mr. Hewfon in the Phil. Tranf. Vol. 

 JjXIII. for 1773, might not in fome meafure have been in- 

 fluenced by a deception of this kind : but, 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 fuch 

 as Mr. Hewfon has defcribed them. 



REMARKS ON THE C0L0UE.S OF NATURAL BODIES. 



Exper. 5. I have already adduced, in illuftration of New- On the colours 

 ton's comparifon of the colours of natural bodies with iho{e°^' ^^^^^^^o^<^^ 

 of-tliin plates. Dr. Wollaflon's obfeyvations on the blue light 

 pf the lower part of a candle, which appears, when viewed 

 through a prifm, to be divided into hve portions. I have 

 lately obferved a limilar inftance, flill more ftrongly marked, 

 in the light tranfmitted by the blue glafs fold by the opti- 

 f ians. This light is feparated by the prifm into feven dif- 

 tin<5l portions, nearly equal in magnitude, but fomewhat 

 broader, and lefs accurately defined, towards the violet end 

 of the fpectrum. The firfl two are red, the third is yellow^. 

 i Hi green, the fourth green, the fifth blue,- the fixth bluilh 

 violet, and the feventh violet. This divifion agrees very 

 nearly with that of the light refleded by a plate of air ^^\^ 

 of an inch in liiicknefs correfponding to the 11th feries of 

 red and the 18th of violet. A fimilar plate of metallic oxide, 

 would perhaps be about Tji's-G ^^ ^^ ^^^^ ^" thicknefs. But 

 it mufl be confeiFed, that there are ftrong reafons for believing 

 the colouring particles of natural bodies in general to be in- 

 comparably fmalier than this; and it is probable that the 

 "linalogy, fuggeflyd by Newton, is fomewhat lefs clofe than 

 ' "'- he 



