C. Hartwell on a Tertiary Rainbow. 57 
College, the thought flashed into my mind that this was a tertiary 
bow. Not recalling the dimensions of such a bow, I measured 
off the heavens as best I could, and judged the radius as seen to 
be about 40°. L have since learned that the radius by calculation 
is 40° 40’, so that my judgment, correct or incorrect, agrees very 
well with the true dimeusions of the bow. 
Having stated these facts to Prof. Snell, and requested a brief 
communication from him, he kindly furnished one, which. I ap- 
pend to this paper. 
“ Amherst College, Aug. 15, 1851. 
Since hearing you give an account of a phenomenon which 
you observed and supposed to be a tertiary rainbow, I have ex- 
amined my notes upon the subject, and am well satisfied that you 
were not mistaken. All the circumstances forbid the supposition 
that it was a halo formed in prisms of ice. You estimated the 
distance of the arcs from the sun to be 40°; this differs but about 
half a degree from the radius of the tertiary bow, as determined 
by calculation. The arcs were seen also in masses of falling rain 
of such limited thickness, that the light might well be supposed 
to have been transmitted through the rain to the eye. 
Though the appearance was not one of remarkable splendor, 
yet you may congratulate yourself, I think, on having witnessed 
a phenomenon of the most rare occurrence. The writer of the 
treatise on Optics, in the English Library of Useful Knowledge, 
speaking of the bows caused by three or four reflections in each 
drop of rain, says, ‘none of these bows, however, have been seen.’ 
Professor Forbes, of the University of Edinburgh, in his learned 
eport on Meteorology to the British Association, 1840, remarks, 
‘These, the ternary, quaternary, &c., rainbows, have been long 
theoretically known, though rarely, if ever, observed in nature. 
The ternary rainbow ought to be about 41° from the sun, but is 
generally stated to be too faint to be visible. 'T'wo observations 
by Bergmann are the only recorded ones I have met with. 
Kemptz observed a ternary amidst the spray of the falls of 
Schauffhausen.’ 
Kemptz, in his course of meteorology, after speaking of the 
bows of the third and fourth order, adds, ‘ but the intensity of 
these two latter is so feeble that they are rarely seen ;’ and puts 
In a note the following quotation, ‘ M. Babinet, when in the most 
favorable circumstances, on Mount d’Or, and on the Canigou, 
vainly endeavored to perceive them. 
It is obvious, as these quotations show, that the phenomenon 
Which you witnessed is as rare as it is interesting. It seems 
to me highly probable -that there is yet no public record of an 
instance in which an American observer has seen a rainbow on 
the same side of the heavens with the sun.” 
Stconp Senses, Vol. XVII, No. 49.—Jan,, 1854. 8 
