G3S OPT 
lated by convenience; for I found (fays the profeffor) a 
receiver of 2200 inches to afford quantities fcarcely one- 
tenth lefs than thofe given by a cafe of the ordinary di- 
raenfions. 
This inftrument was firft conftructed in the autumn of 
1797. It not only meafures the direct rays of the fun, but 
the reflected iight of the fky, for which it is principally 
defigned. It is fenfible to every fluctuation of the atmo- 
fphere, marks the progrefs and decline of the light of day, 
and the periodic increafe and diminution of the bright- 
nefs of the year. It enables us likewife to eftimate other 
light, fuch as the flame of a candle. By comparing two 
photometers, it is eafy to determine the relative proper¬ 
ties of different-coloured fubftances, in reflecting, abforb- 
ing, and tranfmitting-, light. In the fame manner, they 
will determine the queftion, whether the particles of light 
are fpread over the prifmatic fpectrum with equal inten- 
fity. By help of this inftrument, too, we can meafure 
the quantity of light tranfmitted through various dia¬ 
phanous bodies, and that refleftedor abforbed at different 
angles of incidence from poliilied or rough furfaces ; in 
fhort, perform with the utmoft facility all thole ingenious 
experiments which have exercifed the fagacity of Bouguer 
and Lambert. Another fet of inquiries, for which the 
photometer is nicely calculated, is to difcover the con¬ 
ducing powers of different fluids for heat. If the glafs 
cafe, for inftance, be filled with a gas of higher conduc¬ 
ing power than common air, the inftrument will be pro¬ 
portionally lefs affeCed by the fame afflux of light, fince 
thofe are the two balancing conditions. With air, too, of 
different denfities, the effeCs are materially different. In 
that way the author has examined various liquids and 
safes, nay, jellies and ice. His experiments on thefe and 
other points are completed, and afford refults which are 
fatisfaCory and important. Fora farther account of this 
inftrument, and of a curious and valuable colleCion of 
faCs eftablifhed by means of it, we refer to Leflie’s Short 
Account of Experiments and Inftruments on the Relation 
of Heat to Air and Moifture; and to his Experimental 
Inquiry into the Nature and Propagation of Heat, 1804. 
Optometer, or Measurer of Sight. 
The optometer is an inftrument invented by Dr. Porter¬ 
field,-and fo named by him, from its ufe in meafuring the 
limits of diftinC vifion, and in determining with great 
exaCnefs the ftrength and weaknefs of fight. See his 
Treadle on the Eye, vol. i. The principles of its con- 
ftruCion have alfo been explained by Dr. Thomas Young; 
and an improved inftrument, for the fame purpofe, has 
been delcribed by him, in the Phil. Tranf. vol. xci. part i. 
Let an obftacle be interpofed between a radiant point R, 
fip-. 11. and any refraCing furface, or lens, CD ; and let 
this obftacle be perforated at two points, A and B, only. 
Let the re fra ft ed rays be intercepted by a plane, lo as to 
form an image on it. Then it is evident, that, when this 
plane, EF, palfes through the focus of refraCed rays, the 
image formed on it will be a fingle point. But, if the 
plane be advanced forwards to GH, or removed back¬ 
wards to IK, the fmall pencils pafflng through the perfo¬ 
rations will no longer meet in a fingle point, hut will 
fall on two diftinC fpots of the plane, G, H, or I, K ; and, 
in either cafe, form a double image of the objeC. Let us 
now add two more radiating points, the one nearer to the 
lens than the firft point, the other more remote ; and, when 
the plane, which receives 'the images, palfes through the 
focus of rays coming from the firft point, the images of the 
fiecond and third points mull both be double, lince the 
plane, EF, will be without the focal diftance of rays 
coming from the farth'eft point, and within that of rays 
coming from the neareft. Upon this principle Dr. Por¬ 
terfield’s optometer was founded. But, if the three points 
be fuppoled to be joined by a lijie, and this line to be 
fomewhat inclined to the axis of the lens, each point of 
the line, except the firft point R, will have a double image; 
and each pair of images, being contiguous to thofe of the 
ICS. 
neighbouring radiant points, will form with them two 
continued lines ; and, the images being more widely fepa- 
rated as the point which they reprefent is farther from the- 
firft radiant point, the lines will converge on each fide to¬ 
wards theimage of this point, and there will interfect each 
other. The fame happens when we look at any objeft 
through two pin-holes, within the limits of the pupil. If 
the objeC be at the point of perfect vifion, the image on 
the retina will be fingle; but, in every other cafe, the 
image being double, we fliall appear to fee a double ob¬ 
jeC; and, if we look at a line pointed nearly to the eye, 
it will appear as two lines, eroding each other in the point 
of perfeC vifion. For this purpofe, the holes may be con¬ 
verted into flits, which render the images nearly as dif¬ 
tinC, at the fame time that they admit more light. The 
number may be increafed from two to four, or more, when¬ 
ever particular inveftigations render it nece-iTary. 
The optometer may be made of a flip of card-paper, or 
of ivory, about eight inches in length, and one in breadth, 
divided longitudinally by a black line, which imill not be 
too llrong. The end of the card mpft be cut, as is fhown 
in fig. i2. in' order that it may be turned up, and fixed in 
an inclined pofition'by means of the fhoulders ; or a de¬ 
tached piece, nearly of this form, may be applied to the 
optometer, as it is here engraved, A hole about half an 
inch fquare muft be made in this part; and the fides fo 
cut as to receive a Aider of thick paper, with flits of dif¬ 
ferent fizes, from a 40th to a 10th of an inch in breadth, 
divided by fpaces fomewhat broader; fo that each obferver 
may choofe that which beft fuits the aperture of his pupil. 
In order to adapt the inftrument to the ufe of prefbyopic 
eyes, the other end muft be furniftied with a lens of four 
inches focal length ; and a fcale muft be made near, the 
line on each fide of it, divided from one end into inches, 
and from the other according to a table annexed ; by 
means of which, not only diverging, but alfo parallel 
and converging, rays, from the lens, are referred to their 
virtual focus. The inftrument is eafiiv applicable to the 
purpofe of afeertaining the focal length of fpeclacles re¬ 
quired for myopic or prefbyopic eyes. Our author, hav¬ 
ing been furnilhed by Mr. Cary with the numbers and 
focal lengths of the glafles commonly made, calculated the 
diftances at which thefe numbers muft be placed on the 
fcale of the optometer, fo that a prefbyopic eye may he 
enabled to fee.at eight inches diftance, by ufing the glafles 
of the focal length placed opposite to the neareft crofting- 
of the lines; and a myopic eye, with parallel rays, by 
ufing the glafles indicated by the number that ftands op¬ 
posite their fartheft eroding. To facilitate the obferva- 
tions, thefe numbers are placed oppoiite that point which 
will be the neareft cm (ling to myopic eyes; but this, upon 
the arbitrary fuppofition of an equal capability or change 
of focus in every eye, which is often far from the truth. 
It cannot be expended, that every perfon, on the firft trial, 
will fix precifely upon that focus which beft fuits the de¬ 
feat of his fight. Few can bring their eyes at pleafure to 
the ftate.of full aft ion, or of perfect relaxation; and a 
power, two or three degrees lower than that which is thus 
'afeertained, will be found fufficient for ordinary purpofes. 
To the fecond table, Dr. P. has added fuch numbers as 
wiil point out the fpeftacles neceflary for a prefbyopic eye, 
to fee at twelve, and at eighteen, inches reipeftively: the 
middle f’eries will perhaps be the moft proper for placing 
the numbers on the fcale. The optometer fliould be ap¬ 
plied to each eye ; and, at the time of obferving, the 00- 
polite eye fliould not be.fhut, but the inftrument fhouid 
lie fereened from its view. The place of inteffeftion may 
be accurately alcer-tained by means of an index Aiding 
along the fcale. The form of the inftrument, when ready 
to be applied, is reprefented at fig. 1 3. 
The Kaleidoscope. Plate XV. 
Ivaleidofcope is compounded of three Greek words ; 
naAo?, beautiful, ei^o?, a form, and cry.07rea, to lee. This 
curious and amufing inftrument, now well known, was 
recently 
