OPT 
the lathe ; but thefe methods, however Ample and expe¬ 
ditious they may be, Ihould never be adopted for forming 
the lenfes of optical inftruments where an accurate fphe- 
ricnl figure is indifpenfable. It is by the hand alone that 
we can perform with accuracy thofe circular and tranf- 
verfe ftrokes, the proper union of which is efiential to the 
production of a fpherical furface. Appendix to Fergufan's 
LeStvrcs, vol. ii. 7 mifon's School of Arts, Part II. if*Ml. 
Trmf. N° 459. 
Multiplying Glasses. 
The multiplying glafs is made by grinding down the 
convex fide Mlt, Plate VIII. fig. 1. of a plano-convex glafs 
AB, into feveral flat furfaces, as hb, hid, dk. An objett 
■C will not appear magnified when feen through this glafs 
by the eye at H ; but it will appear multiplied into as 
many different objects as the glafs contains plane fur- 
faces. For, fince fays will flow from the object C to all 
parts of the glafs, and each plane furface will refraCt thefe 
rays to the eye, the fame objeCt will appear to the eye 
in the direction of the rays which enter it through each 
furface. Thus a ray gt'H, falling perpendicularly on the 
middle furface, will go through the glafs to the eye with¬ 
out fuffering any refrafition ; and will therefore (how the 
objeCt in its true place at C : whilft a ray ah, flowing from 
the fame objeCt, and falling obliquely on the plane fur¬ 
face bh, will be refraCted in the direction be, by palling 
through the glafs ; and, upon leaving it, will go on to the 
eye in the direction eH ; which will make the fame objeCt 
C appear alfo at E, in the direction of the ray He, pro¬ 
duced in the right line Hen. And the ray cd, flowing 
from the objeCt C, and falling obliquely on the plane fur¬ 
face dk, will in the fame way be refraCted to the eye at H; 
which will caufe the fame objeCt to appear at D, in the 
direction H Ifm. If the glafs be turned round the line 
as an axis, the objeCt C will keep its place, becaufe the 
furface bid is not removed ; but all the other objeCts 
will feem to go round C, becaufe the oblique planes, on 
which the rays abed fall, will turn round by the motion of 
the glafs. 
Camera Obscura. 
The Camera Obfcura having already been fully de- 
feribed under that article in vol. iii. we fliall at prefent 
only direct the reader’s attention to an improvement 
which has lately been made upon this amufing inftrument. 
« The improvements (fays Dr. Brewfter) which have been 
made upon the camera oblcura fince its firft invention, 
regard chiefly its external form ; and no attempts have 
been made to increafe the brilliancy and diftinCtnefs of 
the image. When we compare the picture of external ob¬ 
jects which is formed in a dark chamber by the objeCt-glafs 
of a common refraCting telefcope, with that which is formed 
with an achromatic objeCt-glafs, we fliall find the difference 
between their diftinCtnefs much lefs than we fliould have 
at firft expeCted. Although the achromatic lens forms 
an image of the minuteft parts of the landfcape, yet, when 
this image is received on paper, thefe minute parts are 
obliterated by the fmall hairs and alperities on its fur¬ 
face, and the effeCt of the picture is very much impaired. 
In the Royal Obfervatory at Greenwich the image is 
received upon a large concave piece of ftucco ; but this 
fubftance does not leem to be more favourable for the re¬ 
ception of images than a paper ground. In order to ob¬ 
viate thefe imperfections, I tried a number of white fub- 
ftances of different degrees of fmoothnefs, and feveral 
metallic furfaces with different degrees of polifh, but did 
not fucceed in finding any furface fuperior to paper. I 
happened, however, to receive the image on the filvered 
back of a looking-glafs, and was furprifed at the bril¬ 
liancy and diftinCtnefs with which external objeCts were 
reprefented. The little fpherical protuberances, however, 
which arife from the roughnefs of the tin-foil, have a ten¬ 
dency to detraCl from the precifion of the image, and 
I c S. 
certainly injure it confiderably, when examined narrowly 
with the eye. In order to remove thefe fmall eminences, 
I ground the furface carefully with a bed of hones which 
I had ufed for working the plain fpecula of Newtonian 
telefcopes. By this operation, which is exceedingly deli¬ 
cate, and may be performed without injuring the mirror, 
I obtained a furface finely adapted for the reception of 
images. The minute parts of the landfcape, when re¬ 
ceived on this fubftance, are formed with fo much preci¬ 
fion, and the brilliancy of the colouringis fo uncommonly 
fine, as to equal, if not furpafs, the images formed in the 
air by means of concave fpecula. Notwithftanding the 
bluifli colour of the metallic ground, white objeCts are 
reprefented in their true colour ; and the verdure of the 
foliage is fo rich and vivid, that the image feems to fur¬ 
pafs in beauty even the objeCt itfelf. On account of the 
metallic Iuftre of the furface, the diftinCtnefs of the image 
will always be greateft when the eye of the obferver is 
placed in the direction of the reflected rays. 
The common portable camera obfcura, which has 
already been deferibed, is neceflarily on a fmall fcale, and 
lias many difadvantages. Thefe difadvantages are com¬ 
pletely remedied in the camera obfcura invented by the 
Rev. Mr. Thomfon of Duddingfton, which is reprefented » 
in figures 2 and 3 of the Plate. A is a metallic or wooden 
ring, in which the four wooden bars, AF, AT, AG, AH, 
move by means of joints at A, and are kept afunder by the 
crofs-pieces BC,DE, which move round Band Das centres, 
and fold up along BA and DA, when the inftrument 
is not ufed. The furface FIGH, on which the image is 
received, confifts of a piece of filk covered with paper. 
It is made to roll up at IH, which moves in a joint at I j 
fo that the whole furface FIHG, when winded upon IH, 
can be folded upon the bar IA. By this means the in¬ 
ftrument, which is covered with green illk coated with 
a black fubftance, may be put together and carried as an 
umbrella. It is ftiown more fully in fig. 3. where A is 
the aperture for placing the lens, and BC a femicircular 
opening for viewing the image. A black veil may be 
fixed to the circumference of BC, and thrown over the 
head of the obferver to prevent the admiflion of any ex¬ 
traneous light.” 
Telescopes. Plate VIII. IX. 
Before the invention of telefcopes, aftronomical obfer- 
vations were fometimes made with tubes, but more gene¬ 
rally with rods (regulae), which had fights fixed at each 
end. Thefe inftruments were generally called dioptra ; 
and the apertures in the fights were ca\\e& foramina diop- 
trorum, not perfpicilla. Ptolemy, in his Almageft, de- 
feribes the inftrument which is generally known by the 
name of his triquetrum. It had lights at each end, with 
apertures in them, which Georgius Trapezuntius, the 
tranflator, calls foramina: he mentions, indeed, no par¬ 
ticular name for this part of the inftrument, but Theon, 
in his Commentary, defcribes the conftrufition of it to be 
fuch, that the whole moon would appear through the 
fights, ev tv S'iottieix. Proclus Diadochus, in his Hypoty- 
polis Aftronomicarum Pofitionum, mentions the dioptra 
of Hipparchus, which was an inftrument of the fame 
kind ; lee Riccioli Almegiftum Novum, vol. i. b. 3. But 
we can have no further doubt upon the fubjedt, if we turn 
to Flamftead’s Hiftoria Caeleftis, vol. iii. p. 97. He there 
fays that “ baud pauci obfervationum Tychonicarum er- 
rores ex nudorum dioptrorom ufu neceflarie confequun- 
tur.” Now it is only neceflary to examine the Aftronomia; 
Inftauratae Mechanica, and we (hall fee that almoft all the 
inftruments which Tycho Brahe ufed are fitted with 
fights, which were fome perforated with fmall holes, and 
fome divided by narrow flits: this part of the inftrument 
he called dioptra, the perforations he called foramina, and 
the flits rimulce. Nicliolfon's Journal, Mar. 1805. 
We have already noticed the claims of Metius and of 
Janfen tp the invention of the telefcope properly fo called. 
We 
