14 PHILOSOPHICAL SOCIETY OF WASHINGTON. 
small pencil of nearly parallel rays falling upon the first prism 
then suffers so much dispersion before reaching» the second prism 
that the latter can no longer reunite the rays of different wave- 
lengths, but it will still render them parallel to the directions they 
had before entering the first prism. Thus all the conditions neces- 
sary for affording distinct vision of the radiant to an eye situated 
behind the second prism are fulfilled, and at the same time the 
wave-length of the light received by the eye can be completely con- 
trolled by the adjustment of the prisms. 
In the experimental apparatus actually constructed two sixty- 
degree prisms were employed, separated by an interval of thirty- 
eight inches. These prisms were fixed relatively to each other, and 
behind the second one a viewing telescope of 6.5 inches focal dis- 
tance and 0.84 of an inch clear aperture was mounted in such a 
way that it could be moved through an arc sufficient to bring rays 
of any desired wave-length to the center of its field. Slits were 
placed immediately behind the first prism and before the objective 
of the viewing telescope. Thus arranged, the instrument gave 
images of the whole sun composed of tolerably homogeneous light ; 
but the image of the surrounding sky was not composed of homoge- 
neous light, and to remedy that defect a somewhat different arrange- 
ment of the slits will be tried. 
This communication was followed by a symposium upon the ques- 
tion, 
WHAT IS TOPOGRAPHY ? 
participated in by Messrs. M. H. Doorrrrix, W. D. Jounson, H. 
G. OapEN, and GILBERT THOMPSON. 
Mr. DoortrrLeE began by declaring his ignorance until recently 
of the subject and his resulting fitness to investigate without preju- 
dice. He then pointed out the origin and development of the terms 
geography, chorography, and topography, referring to the death of 
the term chorography at the ripe old age of 1,500 years or there- 
abouts and the alteration with time of the meaning of the words 
geography and topography. The early meaning of topography, 
description of places, appears to be obsolete or obsolescent. The 
existing confusion concerning the meaning of the word topography 
was made strikingly manifest by quotations from dictionaries. 
