442 Mr EARNSHAW, ON THE DIFFRACTION OF AN OBJECT-GLASS, &c. 



pression (C) is sin- i"t^_ cos0J, which denotes spectra of the same charac- 

 ter as are exhibited in Fraunhofer's gratings. 



If the distance of HM from OA be an odd multiple of — , then 



the principal factor in the expression (C) is cos° [ — — — cos0), which 



represents spectra of the same kind as before, but intermediate to them 

 in position. 



For a given value of m, r is greater for red than for violet 

 coloured light, and consequently the spectra will have their red ends 

 outwards, that is, farthest from the centre of the screen. 



What is here said of 3IH referred to OA, is equally true of G3I 

 referred to OH: and what is said of the portion of the image within 

 AOR, is true of the portion within BOB; the line OR bisecting the 

 angle A OB. 



In Sir J. Herschers experiment no spectra of this nature were seen, 

 but with strong sun light they are very distinctly visible, and form to 

 the six bright silvery rays a very beautiful appendage. In fact, on ac- 

 count of the remarkable symmetry of its parts, and of the great extent 

 and extreme narrowness and whiteness of its six principal rays, which 

 stretch completely across the field of view ; and on account of the 

 number and geometrical arrangement of the coloured spectra, this ex- 

 periment is inferior in beauty and splendour to very few of all those 

 that have been exhibited in illustration of the science of Physical 

 Optics. 



S. EARNSHAW, 



