I c s. 
652 
OPT 
creation of beautiful forms: indeed, from its conftruc- 
tion, it is quite incapable of producing any of the fin- 
gular efteCts of the kaleidofcope. As, however, the 
fimilarity between the two inftruments is maintained by 
many perfons, either from ignorance or intereft; in order, 
therefore, to render that juftice to Dr. Brewlter which 
to us appears his due, we give the following ftatement of 
the differences between the two inftruments, upon the 
fuppofition of their both being applied to geometric lines 
upon paper. 
1. In Bradley’s inftru- 
ment, the length is lefs than 
the breadth of the plates. 
2. Bradley’s inftrument 
cannot be ufed with a tube. 
3. In Bradley’s inftru¬ 
ment, from the erroneous 
pofition of the eye, there is 
a great inequality of light in 
the feCtors, and the laft fee- 
tors are fcarcely vifible. 
4. In Bradley’s inftru¬ 
ment, the figure confifts of 
elliptical and confequently 
unequal feCtors. 
5. In Bradley’s inftru¬ 
ment, the unequal feCtors 
do not unite, but are all fe- 
parated from one another 
by afpace equal to the thick- 
nefs of the mirror-glafs. 
6. In Bradley’s inftru¬ 
ment, the images reflected 
from the firft furface inter¬ 
fere with thofe reflected from 
the fecond, and produce a 
confufion and overlapping 
of images entirely inconfif- 
tent with fymmetry. 
7. In Bradley’s inftru¬ 
ment, the defetts in the 
junction of the plates are all 
rendered vifible by the erro¬ 
neous pofition of the eye. 
1. In the kaleidofcope, the 
length of the plates muft be 
four, five, or fix, times their 
breadth. ~ 
2. The kaleidofcope can¬ 
not be ufed without a tube. 
3. In the kaleidofcope, the 
eye is fo placed, that the 
uniformity of light is a 
maximum, and the laft fee- 
tors are diftinCtly vifible. 
4. In the kaleidofcope, all 
the feCtors are equal, and 
compofe a perfeCt circle, and 
the picture is perfectly fym- 
metrical. 
5. In the kaleidofcope, 
the equal feCtors all unite 
into a complete and perfect¬ 
ly fymmetrical form. 
6. In the kaleidofcope, the 
fecondary reflections are en¬ 
tirely removed, and there¬ 
fore no confufion takes 
place. 
7. In the kaleidofcope, the 
eye is fo placed, that thefe 
defects of junction are in- 
viiible. • 
done, we may draw in every one of the divifions a figure, 
at our pleafure, either for garden-platts or fortifications 5 
as for example, in fig 18, we fee a circle divided into fix 
parts, and upon the divifion marked F is drawn part of 
a defign for a garden. Now, to fee that defign entire, 
which is yet confufed, we muft place our glaffes upon 
the paper, and open them to the fixth part of the circle; 
i. e. one of thdm muft Hand upon the line b, to the 
centre, and the other muft be opened exaCtly to the point 
e; fo (hall we difeover an entire garden-platt in a circular 
form, (if we look into the glaffes,) divided into fix parts, 
with as many walks leading to the centre, where we fhal-1 
find a bafin of an hexagonal figure. 
“ The line A, where the glaffes join, ftands immedi¬ 
ately over the centre of the circle, the glafs B ftands upon 
the line drawn from the centre to the point C, and the 
glafs D ftands upon the line leading from the centre to 
the point E. The glaffes, being thus placed, cannot fail 
to produce the complete figure we look for; and fo, 
whatever equal part of a circle you mark out, let the line 
A ftand always upon the centre, and open your glaffes 
to the divifion you have made with your compaffes. If, 
inftead of a circle, you would have the figure of a hexa¬ 
gon, draw a ftraight line, with a pen, from the point e to 
the point b, and, by placing the glaffes as before, you 
will have the figure defired. 
“ So likewife a pentagon may be perfectly reprefented, 
by finding the fifth part of a circle, and placing the 
glaffes upon the outlines of it; and the fourth part of a 
circle will likewife produce a fquare, by means of the 
glaffes, or, by the fame rule, will give us any figure of 
equal fides. I eafily fuppofe that a curious perfon, by a 
little practice with thefe glaffes, may make many im¬ 
provements with them, which perhaps I may not have 
yet difcovered, or have, for brevity fake, omitted to de- 
feribe.” 
We owe an apology to Dr. Brewfter,- perhaps, for hav¬ 
ing tranferibed fo much from his work. We have been 
led on infenfibly, by our anxiety to explain the principles 
and conftruCtion of his curious invention, into a longer 
detail than we intended. But feveral of the modifica¬ 
tions of the inftruments we have ftill left unexplained; 
and we can affure our readers, that we have by no means 
exhaufted the fources of amufement and inftruction which 
they will find on a perufal of the doctor’s “Treatife on 
the Kaleidofcope; Ebinb. 1819.” 
To which may be added, that profeffors Playfair of 
Edinburgh, and Pifitet of Geneva, and the celebrated 
Mr. Watt, have each of them borne teftimonv to the dif- 
fimilarity of the two inftruments, and to the unqueftion- 
able claim which Dr. Brewfter has to the invention of the 
kaleidofcope. But, as'fo much has been faid of the 
claims of Mr. Bradley, who was profeffor of botany in 
the univerfity of Cambridge, we fhall prefent (fee fig. 18) 
an exact copy of the mirrors ufed by him; and his me¬ 
thod of ufing them we fhall give in his own words from 
his work, now fcarce, entitled New Improvements in 
Planting and Gardening, 8vo. Lond. 1717. 
“ We muft choofe two pieces of looking-glafs, (fays 
he,) of equal bignefs, of the figure of a long fquare, five 
inches in length, and four in breadth; they muft be 
covered on the back with paper or filk, to prevent rub¬ 
bing off the filver, which would elfe be too apt to crack 
off by frequent ufe. This covering for the back of the 
glaffes muft be fo put on, that nothing of it may appear 
about the edges on the bright fide. 
“The glaffes being thus prepared, they muft be laid 
face to face, and hinged together, fo that they may be 
made to open and fhut at pleafure, like the leaves of a 
book ; and now, the glaffes being thus fitted for our pur- 
pofe, I fhall proceed to explain the ufe of them. 
“ Draw a large circle upon paper; divide it into three, 
four, five, fix, feven, or eight, equal parts; which being 
PROPERTIES of OPTICAL INSTRUMENTS. 
Plate XVI. 
Of Hadley's Quadrant. 
Upon the radii DC, EC, of the quadrant OEC, fig. t, 
and at right angles to its plane, are fixed two plane reflec¬ 
tors A, B, whole furfaces are parallel when the index D, 
on the movable radius CD, is brought to O; and confe¬ 
quently the arc OD will meafure their inclination when 
the movable radius CD is in any other fituation. The 
wdiole furface of the glafs B is not quickfil vered, apart 
of it being left tranfparent, that objects may be feen di¬ 
rectly through it, and by rays which pafs dole to the 
quickfilvered, or reflecting, part. 
When the angular diftance of two objeCts, S, Q, is to 
be taken, the quadrant is held in fuch a pofition that its 
plane paffes through them both; and the radius CD is 
moved till one of them S is feen, after two reflections of 
the incident ray SA, in the direction HQ; and the other, 
Q, by the direCt ray QH, in the fame line; that is, till the 
objeCts apparently coincide. Then, if SA be produced 
till it meets QH in H, the angle SHQ, contained between 
the firft incident and laft reflected ray, is equal to twice the 
angle of inclination of the two refleCtors; therefore, the 
angular diftance of the two objeCts is meafured by twice 
the arc DO. The method of adjufting this inftrument 
may be feen in Mr. Vince’s Practical Aitronomy, p. 8. 
