periodical colours produced hy grooved surfaces. 49 



These colours are obviously those of the reflected rings in 

 thin plates. By turning the steel plate round in azimuth, the 

 very same colours are seen at the same angles of incidence, and 

 they suffer no change either by varying the distance of the lu- 

 minous aperture, or the distance of the eye of the observer. 



I now examined various other specimens which possessed 

 the same property. In some there were three orders of co- 

 lours, in others two, and others one, while in some only one or 

 two tints of the first order were developed. These different 

 effects are more minutely detailed in the following Table. 



No. of grooves Orders and portions of orders of colours developed 



in an inch, from 90° up to 0° of incidence. 



500 Citron yellow of the first order. 



6^5 One complete order, and up to reddish yellow of the 



second order. Colours very dilute. 

 1000 Four complete orders of colours. 



1000 One complete order, together with blue green and yel- 

 lowish green of the second order. 

 1250 One complete order, together with blue and bluish 

 green of the second order. Colours exceedingly 

 faint and diluted. 

 SOOO One complete order, together with blue green and 



greenish yellow of the second order. 

 2000 on wax. One complete order, together with greenish 

 yellow of the second order. 



2000 — One complete order, together with gamboge 



yellow of the second order 



S500 One complete order, together with the full blue 



of the second order. 

 3333 Gamboge yellow of the first order. 



NEW SERIES, VOL. II. NO I. JAN. 1830. D 



