394 C. D. Walcott—Nature of Cyathophycus. 
In 1874 all astronomers hoped and believed that the transit 
of Venus which occurred in December of that year would give 
the solar parallax within 0’’01. These hopes were doomed to 
disappointment, and now, when we are approaching the second 
transit of the pair, there is less enthusiasm than there was eight 
years ago. Nevertheless, the astronomers of the twentieth 
century will not hold us guiltless if we neglect in any respect 
the transit of 1882. Observations of contacts will doubtless be 
made in abundance, but our efforts should not cease with them. 
We have seen that the probable error of a contact observation 
is +0”-15, that there may always be a doubt as to the phase 
observed, and that a passing cloud may cause the loss of the 
transit. On the other hand, the photographic method cannot 
be defeated by passing clouds, is not liable to any uncertainty 
of interpretation, seems to be free from systematic errors, and is 
so accurate that the result from a single negative has a probable 
error of only +0’55. If the sun is visible for so much as fif- 
teen minutes during the whole transit, thirty-two negatives can 
be taken, and they will give as accurate a result as the observa- 
tion of both internal contacts. In view of these facts, can it be 
doubted that the photographic method offers as much accuracy 
as the contact method, and many more chances of success ? 
The transit of 1882 will not settle the value of the solar 
parallax, but it will contribute to that result, directly as a 
must rest. As our knowledge of the earth’s mass may 
made to depend upon quantities which continually increase 
with the time, it will ultimately attain great exactness, and 
then the solar parallax will be known with the same exactness. 
Long before that happy day arrives the present generation of 
astronomers will have passed over to the silent majority, but 
not without the satisfaction of knowing that their labors will 
contribute to that fullness of knowledge which shall be the 
heritage of their successors. 
Washington, D. C., October, 1881. 
Art. LIIL—QOn the Nature of Cyathophycus ; by C. D. Waucort. 
sponges,+ it was instantly suggested that Cyathophycus was 
probably a member of the same group. special effort was 
* Trans. Albany Institute, vol. x, 1879. 4+ This Journal, xxii, July, Aug., 1881. 
