74 A C tt 
fun until they have taken new root; observing alfo not to 
water them too much at firft, efpecially if the earth in which 
they come over is moift; becaufe too much water is very 
injurious to the plants before they are well rooted, but 
afterward they muft be frequently refrefhed with water in 
warm weather; and they muft have a large fhare of air 
admitted to them, otherwife their leaves will be infefted 
with infeCts, and become foul; in which cafe they muft 
be wafhed with a fponge .to clean them, without which tire 
plants will not thrive. In the winter thefe plants muft be 
placed in the warmeft ftove, and in cold weather they 
fhould have but little w-ater given to them, though they 
nmft be frequently refrefhed when the earth is dry; efpe¬ 
cially if they retain their leaves all the winter, they will 
require a greater (hare of water than when they drop their 
leaves; fo that this muft be done with dilcretion, accord¬ 
ing to the ftate in which the plants are. As thefe plants 
grow in magnitude, they fhould be flufted into pots of a : 
larger fize, but they muft not be over-potted, for that will 
infallibly deftroy them. . 
ACHROMATIC, adj. in optics, without colour; a term 
which, it feems, w’as firft ufed by M. de la Lande, in his 
aftronomy, to denote telefcopes of a new invention, con¬ 
trived to remedy aberrations and colours. See Aberra¬ 
tion and Telescope. 
Achromatic Telescope, a fingular fpecies of re¬ 
fracting telefcope, faid to be invented by the late Mr. John 
Dollond, optician to the king, and fince improved by his- 
fon Mr. Peter Dollond, and others. 
Every ray of light pafling obliquely from a rarer into a 
denfer medium, changes its direction towards the perpen¬ 
dicular ; and every ray pafling obliquely from.a denfer in¬ 
to a rarer medium, changes its direction from the-per- 
pendicular. This bending of the ray, caufed by the 
change of its direction, is called its refraCtion; and the- 
quality of light which fubjeCts it to this refraCtion, is call¬ 
ed its refrangibility. Every ray of light, before it is re- 
fraCted, is white, though it confifts of a number of com¬ 
ponent rays, each of which is of a different colour. As 
foon as its refraCted, it is feparated into- its component 
rays, which, from that time, proceed diverging from each 
other, like rays from a center: and this divergency is 
caufed by the different refrangibility of the component 
rays, in fuch fort, that the more the original or compo¬ 
nent ray is refraCted, the more will the compound rays di¬ 
verge when the light is refraCted by one given medium only. 
From hence it has been concluded, that any tw o diffe¬ 
rent mediums that can be made to produce equal refrac¬ 
tions, will neceffarily produce equal divergencies : whence 
it fhould alfo follow, that equal and contrary refractions 
ftiould not only deftroy each other, but that the diver¬ 
gency of the colours caufed by one refraCtion fhould be 
corrected by the other; and that to produce refraCtion that 
would not be affeCted by the different refrangibility of 
light, is impoflible. 
But Mr. Dollond has proved, by many experiments, 
that thefe conclufions are not well founded; from which 
experiments it appeared, that a ray of light, after equal 
and contrary refractions, was ftill fpread into component 
rays differently coloured; in cither words, that two diffe¬ 
rent mediums may caufe equal refraction, but different di¬ 
vergency ; and equal divergency, with different refraCtion. 
It follows therefore that refraCtion may be produced, 
which is not affeCted by the different refrangibility of 
light. In other words, that, if the mediums be different, 
different refraCtions may be produced, though at the fame 
time the divergency caufed by one refraCtion fhall be ex¬ 
actly counteracted by the divergency, caufed by the other; 
and fo an cbjeft may be feen .through mediums which, 
together, caufe the rays to converge, without appearing 
of different colours. 
This is the foundation of Mr. Dollond’s improvement 
of refraCting telefcopes. By fubfequent experiments he 
found, that different forts of glafs differed greatly in their 
refractive qualities,. with refpeCt to the divergency of co- 
X C H ' 
lours. . He found that crown glafs caufes the leaft diver¬ 
gency, ' and white 'flint the moft, when they are wrought 
into forms that produce equal refraCtions. He ground a 
piece of white flint. glafs into a wedge, whofe. angle was 
about 25 degrees; and a piece of crown glafs to another, 
whofe angle was about 29 degrees; and thefe he found 
refraCted nearly alike, but that their divergency of co¬ 
lours was very different. 
He then ground feveral other pieces of crown glafs to 
wedges of different angles, till he got one that was equal, 
in the divergency it produced, to that of a wedge of flint 
glafs of 25 degrees; fo that when they were put together, 
in fuch a manner as to refract in contrary directions, the 
refraCted light was perfectly free from colour. Then, 
meafuring the fractions of each wedge, he found that that 
of the white flint glafs was to that of the crown glafs 
nearly as two to three. And hence any two wedges, made 
of thefe two fubliances, and in this proportion, would, 
when applied together fo as to refraCt in contrary direc¬ 
tions, refraCt the light without any effeCt arifing from,the- 
different refrangibility of the component rays. 
Therefore, to make two fpherical glades that refraCt 
the light in contrary directions, one muft be concave, and 
the-other convex-; and as the rays, after pafling through 
both, muft meet in a focus, the excels- of the refraCtion 
muft be in the convex one : and, as the convex is to refract 
moft, it appears from the experiment that it muft be made 
of crown glafs; and, as the concave is to refraCt leaft, it 
muft be made of white flint. 
And farther, ■ as the refraCtion of fpherical glafles are in 
an inverfe ratio of their focaf diftances, it follows that the' 
focal diftances of the two glafles fhould be in the ratio 
of the refraCtions of the wedges; for, being thus pro¬ 
portioned, every ray of light that pafles through this com¬ 
bined glafs, at whatever diftance from its axis, will corr- 
ftantly be refraCted by the difference between two con¬ 
trary refraCtions, in the proportion required ; and there¬ 
fore the effeCt of the different refrangibility of light will, 
be prevented. 
The removal of this impediment, however, produced 
another: for the two glafles, which were thus combined, 
being fegmenls of very deep fpheres, the aberrations from 
the fpherical-furfaces became fo conliderable, as greatly 
to difturb the diftinCtnefs of the image. Yet confidering 
that the furfaces of fpherical glafles admit of great vari¬ 
ations, though the focal diftance be limited, and that by 
thefe variations their aberration might be made more or 
lefs at pleafure, Mr. Dollond plainly faw that it was pofli- 
ble to make the aberrating of any two glafles equal; and 
that, as in this cafe the refraCtions of the two glafles were 
contrary to each other, and their aberrations being equal, 
thefe would deftroy each other. 
Thus he obtained a perfeCt theory of making objeCt 
glafles, to the apertures of which he could hardly perceive 
any limits; for, if the praCtice could come up to the theo¬ 
ry, they muft admit of apertures of great extent,;ar\d,con- 
fequently bear great magnifying powers. 
The difficulties of the practice are, however, ftill very 
confiderable. For firft, the focal diftances, as well as the 
particular furfaces, muft be proportioned with the utmoft 
accuracy to the denfitiesand .refracting powers of theglaf- 
fes, which vary even in the fame fort of glafs, when made 
at different-times. Secondly, there are four furfaces to 
be wrought perfectly fpherical. However, Mr. Dollond 
could conftr.uCt refracting telefcopes upon thefe principles,, 
with fuch apertures and magnifying powers, under limit¬ 
ed lengths, as greatly-exceed any that were produced be¬ 
fore, in-forming the images of objeCts bright, diftin.Ct, 
and uninfeCted with colours about the edges, through the 
whole extent of a very large field or compafs of view ; of 
which he has given abundant and undeniable teftimony. 
See Telescope. 
There has lately appeared in the Gentleman’s Magazine 
(1790, pa. 890) a paper on the refraCting telefcope, by an 
author whQ figns Veritus } ,in which the invention is afcribed 
to 
