as a?i experimentum criicis as to the nature of Light. 383 



trical on each side of it, although the whole appeared misty 

 and without that darkness in the intervals which I had found 

 in the previous experiments. This continued whatever pains 

 I took to keep every part of the apparatus in adjustment. 

 The dullness and mistiness of the latter phaenomenon led me 

 to conclude that there could be no doubt but the former was 

 the normal result. I however tried the experiment again 

 with every care in the former circumstances, and found the 

 same result as formerly. 



In the Number of Phil. Mag. for April, 1833, p. 279, I 

 stated, " The result of considerable experience with me is, 

 that it may be seen both black and white, though with me it 

 has much oftener been the former, especially when the bands 

 have been well defined," &c. &c. 



In the November Number for 1833, p. 342, I said, "I 

 shall consider it extremely important to determine whether, 

 when an achromatic lens of short focus is used to form the 

 luminous point, the central band of direct interference given 

 by two mirrors is black, as it has appeared to me, and to 

 several friends to whom I have shown it, when adequately 

 tried with a common lens," &c., &c. 



I have in the papers from which the above extracts are 

 taken, publicly stated my difficulties without reserve, and 

 my ideas of the requirements for a decisive mode of expe- 

 rimenting. 



A mode of experimenting has been adopted, (where or 

 with whom the discovery originated I do not know,) which 

 is stated to give the central band white. It is this : the 

 image of the sun formed in the focus of a lens is made 

 to fall on a small aperture in a thin plate of metal ; the 

 light passing through the aperture falls on the two mirrors, 

 and the aperture thus illuminated is called the luminous 

 origin. I am informed, that when the rays falling on the 

 two mirrors are those which pass obliquely through the 

 aperture, then the central band is most distinctly a white 

 one; and it has been argued that this must be the normal 

 way of trying the experiment, inasmuch as the rays of all 

 colours will have accurately a common origin on the edge 

 of the aperture. 



It has been with me a subject of frequent study to find 

 out a method of trying the experiment which must give a 

 result not to be disputed; and I feel confident the appa- 

 ratus described in this paper fulfils every desideratum. 

 The points kept in view have been to employ only such 

 parts as are essential in the simplest form of the experi- 

 ment; and thus in dispensing with the mirror which is 



