to 
REFLECTOR AND PORTRAIT LENS IN 
CELESTIAL PHOTOGRAPAY. 
14 has been much discussed recently, whether the 
reflector is preferable to a portrait lens in celestial 
photography. It may be known to the readers of 
NATURE that I am interested in this subject, since I 
believe I was the first to use the portrait lens for the 
purpose of seeking for large and wide-spread nebule. 
No doubt the reflector has many advantages over the 
portrait lens—advantages which Dr. Roberts has often 
dwelt upon. In consequence of the small absorption of 
light, the lack of the different surfaces, and the absolute 
correction for chemical rays, the focal pictures with the 
reflector must theoretically be much better than with the 
doublet. The two last-named points are especially effec- 
tive; for the star discs are made much smaller and increase 
much slower than with the portrait lens. I have shown 
dyeamiat! RF 
[APRIL 22, 1897 
absorption of light in the glass of the lens, in the latter 
the discs of images are larger, and therefore not so 
intense. But the reflecting power of the mirror is always 
soon diminished through the influence of oxidation, and 
therefore the reflector does not surpass the portrait lens 
practically as much as would be expected. 
The advantages which exist for small reflectors over 
small portrait lenses increase accordingly to the dia- 
meters; the absorption in the portrait lens becomes 
rapidly greater and the sharpness less, and thus the /arge 
mirror will surpass ezcore plus the /arge doublet. 
In spite of this, the portrait lens is much superior to 
the reflector for the work of seeking and charting feeble 
and extended nebulosities. 
The lens takes a much larger field of the sky than the 
mirror. This is known to be the reason why the portrait 
lens should be used exclusively for the photography of 
minor planets, of comets, for making charts, and especially 
Fic. 1.—The nebulz of Orion. 
that the lenses themselves are the reason for the increase 
of star discs in the film (Phofogr. Corresp., 1892), there- 
fore I fully understand the advantages of the reflector. 
With my 6-inch Voigtlander portrait lens, the discs of 
the smallest stars in the Milky Way have a diameter of 
6 to 8 seconds of arc; I think with a good reflector such 
discs may have a much smaller diameter. 
I do not know the diameter of the smallest star discs on 
plates taken with Dr. Roberts’ 20-inch reflector ; we may 
believe, however, that in his best pictures the sharpness 
of the image is much greater than with my 6-inch portrait 
lens. 
As to the light-gathering power—regarding the pro- 
portion of aperture to focal length the same in the two 
cases—there are two reasons why it is greater with the 
reflector than with the portrait lens. Besides the greater 
NO. 1434, VOL. 55] 
or general views of the Milky Way. This last point 
has been taken up (Monthly Notices, R.A.S., vol. lvi. 
No. 1) by Prof. Barnard against Dr. Roberts. Barnard 
shows that the portrait lens is far better adapted to give 
the general structure of the Milky Way than the mirror, 
in which the field is so small. 
I was sure of this from the beginning, and afterwards 
all my plates showed that there is still another reason 
for preferring the portrait lens to the reflector; a reason 
which, depending likewise upon the large field, would 
alone have decided me to use even a big portrait lens 
instead of a reflector. : 
When photographing regions of sky covered with 
feeble and extended nebulosities, only feeble and ex- 
tended darker parts are obtained upon the plate. These 
can only be seen if there are besides these parts other 
