106 
nearly any {pectacle glafs can be made to 
furround the eye, in the manner of a glo- 
bular tturface, the more nearly will every 
part of it be at right angles to the line of 
fight, the more uniform will be the power 
of its diffezant parts, and the more com- 
pletely wili the indiftin€tneis of lateral ob- 
jects be avoided.” 
Now, this inference, upon a little con- 
fideration by any perfon ever fo little ac- 
quainted with the principle of optics, 
will be manifedly erroneous, and quite ir- 
relevant as to the properties of {pegtacle- 
glaffes, or other lentes. By a thin, hollow, 
giais ipherc, the rays to an eye in its cen- 
tre will certainly not fenfibly be refracted ; 
and more perfectly fo will be the rays 
from objeéts through a parallel or true 
ground plane of glais placed before the 
eye, but with this advantage of the plane, 
that a fenfible thicknefs wiil occafion no 
refractive power, when the fame, in a 
giobe, would aét Jike a concave Jens, and 
forme what diminish the appearance of ob- 
jects. Iv is not by the geometrical shape 
of the lens that vilion, or the purpofes of 
iniirumenis, are aniwered, but by the 
refradive power, produced entirely by 
the proportions of curvatures, or radii of 
curvatures, given to their form. 
A meaiicus lens is not morethe fegment 
of a globe, as implied by Dr. Wollafton, 
than a convex lens, but a lens, compoled of 
the furfaces of two {pheres of different ra- 
dit. When the radius of the exterior 
curve exceeds that of the interior, it aéts 
ike a concave lens, and diminithes ob- 
j<&ts. When the radius of the exterior 
eurve is lefs than that of the interior, it 
has the properties of a convex lens, and 
magn fies; but with this difference in 
Both cafes, that, having a greater furface, 
it produces a grea‘er degree of aberration, 
Both of the longitudinal and lateral kinds, 
and confequently more indiftinétnels to- 
wards its periphery than any of the other 
kind of lenfes of the fame foci and dia- 
meter. ‘The truth of this may be feen 
in the optical works of Smith, Emerfin, 
Marite, &c. A practical proof of this 
may be had by the following experiment : 
Take a meniicus of about four inches 
politive focus, and about the fize of a 
ipeciacle gia!s, and alfo a‘tommon convex 
ipeftaele gies of the fame focus and dia- 
meter, by night, ia a room, wich a lighied 
candle at a diitance; hold them about 
four inches and a quarter from a white 
wainfcot or wall, before the candle ; 
there wiil be refraGted to a focus thereon 
two bright inverted images of the flame 
and candle ; but, about the one formed 
Mr, Jones on Spectacle Glaffes— Homer. 
{ March al, 
by the menifcus, there wili bea {mall, 
faint, white circle of aberration, and 
what on the face as a fpetacle-glafs pro- 
duces the peripheretical indiftinétnels of 
objedts, and, by a candle, the prilmatic 
evlours about its flame. Such glaring 
objecis feen obliquely muft be allowed to 
be detrimental to the fight. 
For the fatisfaétion of any intelligent 
perfon, I have conftru&ted a hand-frame, 
confifiing of a double convex, plano-con- 
vex, menilcus, and two plano-convexes to- 
gether, all of the fame focus, four inches, 
and which will afford an ecular procf of 
the inferiority of the meni cus to the other 
form for the purpofes of fpetacles or 
any other infruments. They may be 
viewed gratis at our manufactory, No. 305 
Holborn. Ihave in my poff-ffion an old 
menilcus-fbaped fpeétacle-glais, which E 
ean prove to have been made a great 
number ot years. Ido not therefore fee 
upon what new principle or contrivance 
his Majetty’s letters patent have been fo- - 
licited. ; 
The menifcus-fhaped lens containing 
the greateft aberration of any other form, 
is the reafon why it has been abandoned 
by all tkilful opticians. I am, Sir, 
Holbcrn, Your’s, &c. 
Feb. 14, 1804. Wwe. Jones. 
To the Editor of the Monthly Magazize. 
SIR, ; 
OUR Magazine is enriched with the 
_ contributions of Aikin and Wake- 
field on the fimilies and the imitations of 
Homer. I do not recoll &, that either of 
thefe accomplifhed critics has controyerted 
the pofiticn, that no fimile occurs in the 
firft book cf the Iliad. In the forty- 
feventh verfe of that firft rhapfody, the 
bord betrays his propenity to his favourite 
figure of comparifon: Pe 
6 Se wedi oma 
In the fecond book of Paradife Loft, 
Milton adopts the ftyle and imagery of 
this pafiage of the liad: 
** Black it ftood as night, 
And fhook a dreadful dart.” 
The Grecian defcribes the wrath 
Apollo; the Britith 
terrors of death. 
In the three hundred ard fifty-ninth 
verfe of the firft book of the Iliad, Thetis 
is defcribed as afcending like mut : 
— its” omiydn: . 
"In the feven hundred and tenth verfe of 
the firft of Paradife Loft, we read, 
. Anon, out of the earth a fabric huge 
Rofe like an exhalation. 
of 
poet delineates the 
Innumerabl S 
