CATALOGUE. PHYSICAl- OPTICS. 



317 



An oblong shadow, surrounded by two luminous and 

 coloured arches : the centre being dark, yellow next, then 

 dark, then a rainbow. Quotes Priestley and others for three 

 parallel cases. 

 Barker. Ph. tr. 1783. 245. 1787. 370. 



Some coronae. 



Stratico on the diffraction of light. Ac. Pad. 



II. 185. 

 Hopkinson and Rittenhouse on inflection 



through cloth. Am. tr. II. 201. Nich. 1. 



13. 

 Comparetti de luce inflexa et coloritus. 4. 



Pad. 1787. R.S. 



Contains some curious experiments, but generally in very 

 complicated circumstances. 

 Brougham on inflection and colours. Ph. tr. 



1796. 227. 1797. 352. Nich. 11. 147. 

 Jordan's observations on light and colours, 8. 



Lond. 1799. 1800. R. I. 



Ace. Nich. IV. 78. 

 Colours produced by distant glasses. Nich. 



II. 312. 



Probably from a slight difference in the thickness of the 

 glasses, the rays twice reflected within the first glass only, in- 

 terfering with the rays twice reflected in the second only. 

 The analogy with the colours of thin plates is wholly 

 foreign to the subject. 



Colours of steel. Nich. IV. 127. 



Young on some cases of the production of 



colours. Ph. tr. 1802. 387- Nich. 8. TV. 



180. 

 Young on the colours of thin plates shown by 



the solar microscope. Journ. R. I., 1.241. 



Nich. 8. III. 283. 

 Young on physical optics. Ph. tr. 1804. 1. 



Nich. IX. 63. 

 Messier on a lunar corona. M. Inst. V. 130. 

 Anthelia. See Glories, Parhelia. 



Description of Dr. Young's Apparatus for exhiliting the 

 Colours of thin Plates, by means of the Solar Micro- 

 . scope. Journ, R, 7. I. 241. 



The colours of thin plates were observed by Boyle and 

 Hooke, and more aecurately analysed by Newton : but lit- 



tle or nothing was added to the account that Newton gave 

 of them, until some attempts were lately made to explain 

 them, and to build at the same time on the explanation, the 

 principal arguments in favour of a new system of light and 

 colours. The phenomena themselves were very little 

 known, except from Newton's description ; it had hap- 

 pened but to few to observe them : and they had never been 

 made conspicuous to a public audience in a form equally 

 beautiful and interesting. * 



It appeared, however, that there would be little difficulty 

 in applying the apparatus for representing opaque objects in 

 the solar microscope, to the exhibition of these colours on a 

 large scale : but several precautions were necessary, in order 

 to obtain the most advantageous representation ; and, these 

 precautions having been completely successful, it may beof 

 some utility to give a detached account of them. 



The colours of thin substances must often have been 

 seen in bubbles of water or of other fluids, and in the filin 

 produced by a drop of oil spreading on water ; they were 

 more particularly observed in the plates of talc, or of selenite, 

 into which those substances readily divide. Sir Isaac 

 Newton made his experiments principally on the colours of 

 soap bubbles, and on those which are produced by the con- 

 tact of two lenses. For inspecting the colours of soapy 

 water, the most convenient method is that of Mr. Jordan. 

 He dips a wine glass into a weak solution of soap, and thea 

 holds it in a horizontal position against an upright substance, 

 for example, a window shutter ; the filrn covering the glars 

 being in a vertical position, the gravity of the fluid tends to 

 make it thicker at the lower part, and it becomes every 

 where gradually thinner and thinner, till at lengtii it bursts 

 at the uppermost point. The colours assume, in this case, 

 the form of hcrizontal stripes, similar to the rings which 

 are to be more particularly described. 



It has been observed by Newton, that the colours thus 

 reflected from a plate of a denser medium, are more vivid 

 than when a plate of a rarer medium is interposed between 

 two denser meiiiums. But the cause of this apparent dif- 

 ference is, probably, the quantity of foreign light that is ge- 

 nerally present in the experiment, reflected as well from 

 the upper surface of the superior medium as frcm-the 

 1 Jwcr surface of the inferior, both these surfaces being often 

 nearly parallel to the surface;; in contact. It becomes 

 therefore desirable to remove this foreign light : this may 

 be done efTectually, by employing one glass in the form of 

 a prism, and coiituig the lower surface of the other with 

 black sealing wax : the light reflected by the oblique surfacc 

 ofthe first is thus thrown into another direction; and the. 

 reflection of the inferior surface of the second is either 

 destroyed or rendered imperceptible. And, with these.pre- 

 cautions, the rings of colours, produced in the reflectediight. 



