252 J. 8. Diller—Fulgurite from Mt. Thielson, Oregon. 
retina may be very long, but that a long exposure should be fol- _ 
lowed by no after-images seems contrary to common experience, 
The results of the experiments described in this paper may 
be briefly summed up as follows. 
1. The study of the duration of color-impressions produced — 
by different portions of the spectrum, confirms, in the main, the 
results reached by Plateau. 
2. The persistence of the retinal image is a function of the 
wave-length producing it, being greatest at the ends of the 
spectrum and least in the yellow. Mg 
8. It decreases as the intensity of the ray producing the | 
image increases. is 
4. The relative duration of the impressions produced by the — 
different spectral colors, is not the same for all ey oe 
5. The duration of the retinal image is in inverse order to the — 
- luminosity of the colors producing it. 
6. Hach wave-length of the visible spectrum produces three 
primary impressions, red, green and violet; of which green dis- 
appears most rapidly and violet is the most persistent. Upon 
the different rates at which these impressions die away depend 
to a great extent the “subjective” tints of moving objects. . 
7. The duration of the retinal image depends upon the length — 
of time during which the eye has been exposed, being very 
long after short exposures and approaching a definite finite 
minimum value as the exposure increases. 
University of Kansas, June, 1884, 
eereectncenee 
ArT. XXXII—Fulgurite from Mount Thieison, Oregon; by 
J. S. DILuer. 
THE occurrence of lightning-tubes (Blitzrdhren) or fulgurites, — 
as they are frequently named, in this country has received little 
attention and it is very rarely that specimens have found thelr 
into our museums. This cannot be due to their sear 
n 
western part of the United States. They deserve special atten- 
~~ because they are the product of an exceptional methodol 
usion. 
The terms fulgurite and lightning-tube are frequently used 
he former be ap 
. 
