OPTICS. 
G71. 
11 Fig. 5. reprefents the feflion of a folid prifmatic piece 
of glafs, within which both the refle&ions requifite are 
offered at the furfaces ab, be, in fuch a manner that the 
ray fg, after being refltded firft at g, and again at h, ar¬ 
rives at the eye in a direftion he, at right angles to fg. 
“ There is another circumftance in this conftrudion 
nsce'flary to be attended to, and which remains to be ex¬ 
plained. Where the reflection was produced by a piece 
of plain glafs, it is obvious that any objects behind the 
glafs (if fufficiently illuminated) might be feen through 
the glafs, as well as the reflected image. But, when the 
prifmatic refledor is employed, fince no light can be tranf- 
mitted diredly through it, the eye mud be fo placed that 
only a part of its pupil may be intercepted by the edge of 
the prifm, as at e, fig. 5. The dirtant objeds will then be 
feen by this portion of the eye, while the paper and pen¬ 
cil are feen paft the edge of the prifm by the remainder of 
the pupil. In order to avoid the inconvenience that 
might arife from the unintentional motion of the eye, the 
relative quantities of light to be received from theobjed, 
and from the paper, are regulated by a fmall hole in a piece 
of brafs, which by moving on a centre at c, fig. 4. is capa¬ 
ble of adjuftmen t to every inequality of light that is likely 
to occur. 
“ Since the fizs of the whole inftrument, from being fo 
near the eye, does not require to be large, I have on many 
accounts preferred the fmalleft flze that could be exe¬ 
cuted with corrednefs, and have had it conftruded on 
fuch a fcale, that the lenfes are only three-fourths of an 
inch in diameter. 
“ Though the original defign and principal ufe of this 
inftrument, is to facilitate the delineation of objeds in true 
perfpedive, yet this is by no means the foie pufpofe to 
which it is adapted; for the fame arrangement of reflec¬ 
tors may be employed with equal advantage for copying 
what has been already drawn, and may thus aflift a learner 
in acquiring at lead: a corred outline of any fubjed. For 
this purpofe, the drawing to be copied fliould be placed 
as nearly as may be at the fame diftance before the inftru¬ 
ment that the paper is beneath the eye-hole ; for in that 
cafe the flze will be the fame, and no lens will be neceflary 
either to the objed or to the pencil. By a proper ufe of 
the fame inftrument, every purpofe of the pentagraph 
may alfo be anfwered ; as a painting may be reduced in 
any proportion required, by placing it at a diftance in due 
proportion greater than that of the paper from the inftru¬ 
ment. In this cafe a lens becomes requifite for enabling 
the eye to fee at two unequal diftances with equal diftind- 
nefs ; and, in order that one lens may fuit for all thefe 
purpofes, there is an advantage in carrying the height of 
the ftand according to the proportion in w'hich the reduc¬ 
tion is to be efteded. 
“ The principles on which the height of theftem is ad- 
jufted will be readily underftood by thofe who are accuf- 
tomed to optical conftderations. For as, in taking a per¬ 
fpedive view, the rays from the paper are rendered pa¬ 
rallel by placing a’lens at the diftance of its principal 
focus from the paper, becaufe the rays received from the 
diftant objeds are parallel; fo alfo, when the objed feen 
by refledion is at fo fhort a diftance that the rays received 
from it are in a certain degree divergent, the rays from the 
paper fliouid be made to have the fame degree of diver¬ 
gency in order that the paper may be feen diftindly by 
the fame eye ; and for this purpofe the lens muft be placed 
at a diftance iefs than its principal focus. The Item of 
the inftrument is accordingly marked at certain diftances, 
to which the conjugate foci are in the feveral proportions 
of 2, 3, 4, &c. to 1 ; fo that diftinft vifion may be obtained 
in all cafes, by placing the painting proportionally more 
diftant. By tranfpofing the convex lens to the front of 
the inftrument, and reverftng the proportional diftances, 
the artift might alfo enlarge his fmaller fketches with 
eyery defirable degree of corrednefs, and the naturalift 
might delineate minute objeds in any degree magnified.” 
3. The graphical perfpedive is an inftrument delcribed 
by Mr. Martin, confiding of a tube containing two con¬ 
vex lenfes, which are placed at twice their focal diftance 
from each other ; and in their common focus is another 
glafs, divided into equal parts with the point of a dia¬ 
mond. Though this inftrument does not magnify any 
objed, yet the angle under which an objed is feen is ea- 
fily known by it; and, fince this angle varies with the 
diftance of objeds, it is eafily applied to the purpofe of 
meafuring inacceflible heights and diftances ; and, fince 
the field of view is divided into equal fquares, it is ufeful 
in drawing the perfpedive appearance of objeds. More¬ 
over, as all foreign light is excluded by the tube in which 
thefe lenfes are inclofed, pidures feen through it have a 
fine relievo, on which account, and alfo becaufe objeds 
appear inverted through it, the images of a camera-ob- 
feura are viewed with it to peculiar advantage. If a lens 
of a greater focal length be fixed at a proper diftance from 
the centre of the tube, this inftrument will be a telefcope, 
and will magnify the prints which are looked at through 
it; and, if a fmall lens be ufed, it will be a microfcope, and 
the fame micrometer will ferve for them both. Martin's 
Optics, chap. xvii. 
4. Mr. Kirby has alfo contrived and deferibed an in¬ 
ftrument that will be ufeful in taking extenfive views, 
Sec. The ruler AB, fig. 6, is 19 inches long, and is gra¬ 
duated into 19 equal parts ; it has a dove-tail groove on 
its upper edge to receive the perpendicular ruler G, 
w'hich has one end fitted to it, fo as to Aide very eafily ; 
this ruler is 14 inches long, and is divided into 14 equal 
parts; and, upon the back fide of it F, is a line drawn ex¬ 
actly in the middle, to which is fixed a filken line with 
a fmall plummet at the end. The ruler AB is fixed by 
two ferews a, c, to two pieces of thin brafs ; and thefe 
pieces of brafs are fixed at the other ends by two ferews 
d, e, to a ftronger piece of brafs bf, which goes clofe to 
the ruler AB, and has a joint at X turning upon a ferew ; 
below this joint is a piece of round brafs about fix 
inches long, which goes into a hole made in the top of 
the ftaff, and may be moved higher or lower, by means of 
a ferew S. CD reprefents part of this ftaff, the whole 
length of which is about three feet; and at the bottom 
is a rank ferew, DE, made of iron, and fixed to the ftaff, 
(fee alfo fig. 9.) HI, fig. 7, is a wire 22 inches long, with 
a ferew at h to go into the hole b ; the piece of brafs wire 
bent into the form ik is fixed to the wire HI by the ferew 
h ; and the part i goes into the hole f in the brafs-piece 
bf, (fig. 6.) The fmall wire KL, (fig. 6.) is about 
twelve inches long, and flatted at K, at which place is a 
little hole above one-eighth of an inch in diameter; this 
wire KL is fitted to the holes, l, m,n, o, which are made 
in the larger wire HI; and it may be placed higher or 
lower by means of a fmall ferew. 
This inftrument is ufed in the following manner : Fix 
a paper upon a drawing-board, as in fig. 8. and divide 
the paper lengthways into 19 equal parts, and perpendi¬ 
cularly into 14 equal parts, making thefe divifions great¬ 
er or fmaller according to your defign. Then take the 
ftaff, and fix it ftrongly in the ground by means of the 
ferew at bottom, and at a convenient diftance from the 
profpefl which you intend to take. After this, put the 
inftrument together as in fig. 9. and fix the ruler AB ex- 
aftly horizontal by means of the plummet on the perpen¬ 
dicular ruler and the brafs joint X; then fix the wire 
KL, fo as to have the eye-hole exaftly level with the ho¬ 
rizon or equal to the height of the eye, and take care to 
have the greateft diftance of the eye-hole from the ruler 
equal to the whole length of the longeft ruler AB, and 
never lefs than the diftance hi. Having thus fixed the 
inftrument, proceed to make the drawing ; look through 
the eye-hole, and then move the perpendicular ruler in 
the groove, till you get one edge exactly again ft fome 
principal obje£t; then will the parts upon the ruler Ihow 
how lugh the objeft is from the bottom of the ruler, i. e. 
from the bottom of the picture; and you will alfo know 
its apparent height; therefore transfer this to the paper 
in 
