138 New Experiments on the Solar Spectrum. 



for a camera obscnra, having an eastern exposure, with a circular 

 aperture in the window shutter three fourths of an inch in diam- 

 eter. When the morning sun is shining bright, say from eight 

 to ten o'clock, I place a glass prism in the beam that enters this 

 opening, and form the prismatic spectrum on the opposite wall, 

 distant ten feet. The direct solar beam affords a brighter spec- 

 trum than when introduced by a mirror, as in the heliostat ; and 

 the distance of ten feet is found to afford a spectrum of conven- 

 ient dimensions for the subsequent experiments. 



It is now only necessary to introduce into the prismatic beam, 

 between the prism and the wall, any reflecting or refracting me- 

 dium, or both combined, and we are surprised with the appear- 

 ance on the wall, around the spectrum, of an endless variety of 

 figures, for the most part perfectly symmetrical, and exhibiting 

 hues more gorgeous than I have ever seen developed by any 

 other means, either in art or nature. Since every change in the 

 reflecting or refracting medium produces a new variety of figures, 

 and a new combination of colors, the exhibition may be end- 

 lessly varied. I will subjoin a few examples, which will be un- 

 derstood to be merely a few terms of an infinite series. 



First, a glass tube an inch in diameter* is held in the prismatic 

 beam in the direction of the plane of the rays. Immediately, 

 the several prismatic colors arrange themselves on the wall in 

 broad circular zones. The circles will be larger as the tube is 

 held farther from the wall ; and, at a given distance, the dimen- 

 sions of the figures may be varied by changing the inclination 

 of the tube, being small when the tube approaches a horizontal 

 direction, and enlarging as the tube is raised towards a perpen- 

 dicular position, until they encompass the entire room. Inequal- 

 ities in the glass serve to diversify each individual zone, still 

 preserving, as in the kaleidoscope, remarkable symmetry of ar- 

 rangement. Thus, we not unfrequently observe the zones che- 

 quered by a gossamer net-work. If the prismatic beam is suf- 

 fered to fall obliquely on the end of the tube, so that a part of 

 the light falls upon the convex and a part upon the concave sur- 

 face, peculiarly agreeable diversities of colors are developed ; and 

 if the tube is bent into the form of a syphon, the two arms pre- 



* A tube of any diameter will show the effect, in a greater or less degree — a 

 common Argand lamp chimney will answer the purpose. 



