[Footnote: By David P. Todd, M.A., from the _Proceedings_ of the
American Academy of Arts and Science.]
In the twentieth volume of the _American Journal of Science_, at page
225, I gave a preliminary account of my search, theoretic and practical,
for the trans-Neptunian planet. I say _the_ trans-Neptunian planet,
because I regard the evidence of its existence as well-founded, and
further because, since the time when I was engaged upon this search,
nothing has in the least weakened my entire conviction as to its
existence in about that part of the sky assigned; while, as is well
known, the independent researches in cometary perturbations by Prof.
Forbes conducted him to a result identical with my own--a coincidence not
to be lightly set aside as pure accident.
That five years have elapsed since this coincidence was remarked, and the
planet is still unfound, is not sufficient assurance to me that its
existence is merely fanciful. In so far as I am informed, this spot of
the sky has received very little scrutiny with telescopes competent to
such a search; and most observers finding nothing would, I suspect,
prefer not to announce their ineffective search.
The time has now come when this search can be profitably undertaken by
any observer having the rare combination of time, enthusiasm, and the
necessary appliances. Strongly marked developments in astronomical
photography have been effected since this optical search was conducted;
and the capacity of the modern dry-plate for the registry of the light of
very faint stars makes the application of this method the shortest and
surest way of detecting any such object. Nor is this purely an opinion of
my own. But the required apparatus would be costly; and the instrument,
together with the services of an astronomer and a photographer, would,
for the time being, be necessarily devoted exclusively to the work.
While, however, the photographic search might have to be ended with a
negative result, in so far as the trans-Neptunian planet is concerned,
there would still remain the series of photographic maps of the region
explored, and these would be of incalculable service in the astronomy of
the future.
In the latter part of the paper alluded to above, I stated the
speculative basis upon which I restricted the stellar region to be
examined; also the fact that between November of 1877 and March of 1878 I
was engaged in a telescopic scrutiny of this region, employing the
twenty-six inch refractor of the Naval Observatory. For the purposes
contemplated I had no hesitation in adopting the method of search whereby
I expected to detect the planet by the contrast of its disk and light
with the appearance of an average star of about the thirteenth magnitude.
A power of 600 diameters was often employed, but the field of view of
this eye-piece was so restricted that a power of 400 diameters had to be
used most of the time. I say, too, that, "after the first few nights, I
was surprised at the readiness with which my eye detected any variation
from the average appearance of a star of a given faint magnitude; as a
consequence whereof my observing book contains a large stock of memoranda
of suspected objects. My general plan with these was to observe with a
sufficient degree of accuracy the position of all suspected objects. On
the succeeding night of observation they were re-observed; and, at an
interval of several weeks thereafter, the observation was again
verified." Subjoined to the original observations are printed these
verifications in heavy-faced type.
In conducting the search, the plans were several times varied in slight
detail, generally because experience with the work enabled me to make
improvements in method. Usually, I prepared every few days a new zone
chart of the region over which I was about to search; and these charts
while containing memoranda of all the instrumental data which could be
prepared beforehand, were likewise so adjusted with reference to the
opposition-time of the planet as to avoid, if possible, its stationary
point. The same thing, too, was kept in mind in selecting the times of
subsequent observation. Notwithstanding this precaution, however, it
would be well if some observer who has a large telescope should now
re-examine the positions of these objects.
Researches in faint nebulæ and nebulous stars appearing likely to
constitute a separate and interesting branch of the astronomy of the
future, it has seemed to me that the astronomers engaged in this work may
like to make a careful examination of some of the stars entered in my
observing book under the category of "suspected objects." The method I
adopted of insuring re-observation of these objects was by the
determination, not of their absolute, but only of their relative,
positions, through the agency of the larger "finder" of the great
telescope. This has an aperture of five inches, a power of thirty
diameters, and a field of view of seventy-eight minutes of arc. Two
diagrams were usually drawn in the book for each of these objects, the
one showing the relation of adjacent objects in the great telescope, and
the other the configuration of the more conspicuous objects in the field
of view of the finder. Adjacent to these "finder" diagrams are the
settings--to the nearest minute of arc in declination, and of time in
right ascension--as read from the large finding-circles, divided in black
and white. The field of view of the finder is crossed by two pairs of
hairlines, making a square of about twelve minutes on a side by their
intersection at the center. The diagrams in all cases represent the
objects as seen with an inverting eye-piece. As the adjustment of the
finder was occasionally verified, as well as the readings of the large
circles, there should be no trouble in identifying any of these objects,
notwithstanding the fact that no estimates of absolute magnitude were
recorded. The relative magnitudes, while intended to be only approximate,
are still shown with sufficient accuracy for the purpose of the research,
and the diagrams are, in general, faithful tracings from the original
memoranda.
[Mr. Todd transcribes the observing book entire.]
* * * * *
[AMERICAN ANNALS OF THE DEAF AND DUMB.]