studies on the subject. I must now bring this paper to a close, although I
had intended including some interesting studies of curved surfaces. There
is, however, matter enough in that subject of itself, especially when we
connect it with the idiosyncrasies of the material we have to deal with, a
vital part of the subject that I have not touched upon in the present
paper. You may now inquire, How critical is this "color test"? To answer
this I fear I shall trench upon forbidden grounds, but I call to my help
the words of one of our best American physicists, and I quote from a
letter in which he says by combined calculation and experiment I have
found the limiting error for white light to be 1/50000000 of an inch, and
for Na or sodium light about fifty times greater, or less than 1/800000 of
an inch. Dr. Alfred Mayer estimated and demonstrated by actual experiment
that the smallest black spot on a white ground visible to the naked eye is
about 1/800 of an inch at the distance of normal vision, namely, 10
inches, and that a line, which of course has the element of extension,
1/5000 of an inch in thickness could be seen. In our delicate "color test"
we may decrease the diameter of our black spot a thousand times and still
its perception is possible by the aid of our monochromatic light, and we
may diminish our line ten thousand times, yet find it just perceivable on
the border land of our test by white light. Do not presume I am so foolish
as to even think that the human hand, directed by the human brain, can
ever work the material at his command to such a high standard of
exactness. No; from the very nature of the material we have to work with,
we are forbidden even to hope for such an achievement; and could it be
possible that, through some stroke of good fortune, we could attain this
high ideal, it would be but for a moment, as from the very nature of our
environment it would be but an ignis fatuus. There is, however, to the
earnest mind a delight in having a high model of excellence, for as our
model is so will our work approximate; and although we may go on
approximating _our_ ideal forever, we can never hope to reach that which
has been set for us by the great Master Workman.

* * * * *

[JOURNAL OF GAS LIGHTING.]

PHOTOMETRICAL STANDARDS.

In carrying out a series of photometrical experiments lately, I found that
it was a matter of considerable difficulty to keep the flames of