104 
is -underftood to be confirmed by repeated 
experiments with prifms, which fhew, 
orare fuppofed to fhew, that each pri- 
mary divifion of colour-making rays, 
‘after its pafflage through one prifm, is 
incapable of farther divifion, in its paf- 
fage through other prifms. Our artift 
maintains, on the contrary, that this 
doétrine is a delufion; and that there 
are only ¢hree primary forts of colours; 
the red, the yellow, and the blue; by 
whofe different intermixtures, all the 
intermediate colours may be formed. 
Now it is a principle of the Newtonian 
philofophy, that, when a fimple caufe 
can be afligned for any phenomenon, we 
re not to have recourfe to one more com- 
plicated: if, by the intermixture of three 
iorts of colour-making rays,’ all the other 
colours may be formed, there 1s reafon- 
able ground for conje€ture, that fome 
circumftance has been hitherto overlook- 
ed in our experiments, which has been 
the bafis of a fundamental error in the 
prefent fyftem of optics. In obferving 
the colours of the rainbow, and the 
effects of a prifm upon light, we cannot 
be infenfible to the gradation of colour 
from each of the three ({uppofed primary 
by our artift) to the next in fucceffion ; 
and it may be poffible that the rays of 
light feparated by the firft prifm are not 
fo-difentangled (if we may make ufe of 
the expreflion) as that fome of the rays 
of one colour may not be fo mixed with 
_thofe-of the next to it, as to form the 
intermediate colour. To give a fimple 
inftance: the fines of refraction of all the 
degrees of red are fuppofed to be within 
77 and 77-4 of thole equal parts into 
fifty of which the fine of incidence in 
glais was divided; the fines of refrac- 
tion of all the degrees of orange are 
within 77 4 and.77- of thofe parts, and 
of yellow, within 77.4 and 774 of the 
fame parts. Now may we not fuppofe 
that, if there were no fuch rays as the 
orange-making rays, that colour would 
neceflarily be produced by the weakened 
red-making rays, and the ftrongeft 
yellow, which may come into contaét 
jooner than is imagined? At any rate, 
the opinion of the worthy artift deferves 
philoiophical. inveftigation; with the 
view of promoting which, I hope jome 
of your readers wil! turn their attention 
to the fubject, and give us the refult of 
the experiments. 
T remain, your’s, &c. 
Feb. 20. A. SEARCH, 
Pedeftrian’ Tour in North Wales. 
PEDESTRIAN Tour rn Nort 
; WALES. 
[ Continued from our laft.] 
THE next day, July 2, we {pent in 
examining the town and neighbour- 
hood of Conway. 
Perhaps the moft perfeé&t model of 
ancient fortification is to be feen at 
Conway. The town is very fmall and 
poor; but entirely furrounded by a lofty 
[March > 
wall and mafly towers, which- have’ 
hitherto yielded but little to the depre- 
dations of time; and on a rocky pro- 
montory, connected with the town, and 
projecting into the river, which is there 
about a mile wide, is feated the caftle. 
Mere defcription can give but a- very 
faint idea of this mighty pile : its com- 
manding fituation, its lofty walls, its 
mafly turrets, its elevated watch-towers, 
the large fpace‘over which it extends, 
can be exprefied only by the powers of 
the pencil. After having admired, and 
taken a hafty fketch of the caftle, we 
crofled the river, and proceeded to 
Ormes-head, which is a very craggy and 
lofty rock of limeftone, projeéting a 
confiderable diftance into the fea. Upon 
the moft inacceffible parts, efpecially 
thofe which overhang the water, innu- 
merable multitudes of- gulls, crows, 
cormorants, and other birds, have taken 
up their abode; and the higheft pro- 
jJections are occupied by the peregrine 
falcon. To the naturalift, therefore, as 
well as the pi€turefque obferver, Ormes- 
head is an objeét well worthy of notice, 
After breakfaft, the following day, 
we fet off for Beaumaris. The road lay 
chielly by the fea-fide, and prefented us 
with many interefting objeéts ; the fhores 
of the ifle of Anglefey, the ifland of 
Prieftholm, and the promontory of 
Ormes-head, formed the moft ftriking 
points of view: fometimes the road 
led us through barren glens, walled in 
with huge craggy mountains; at other 
times, from a bold elevation, we were 
prefented with an expanfe of fea, ¢lear 
as glafs, and {mooth as the furface of a 
mirror. The famous pafs of Pexmaen- 
maur, conduéted us round the moft 
northern promontory of Caernarvonfhire, 
and then let us down by degrees on the 
banks of the Menai, the firais which 
feparates the Iile of Anglefey from the 
reft of Wales: the. pafs of Penmaen- 
maur is a fine road, winding round e - 
rocky promontory of the fame name; 
the mountain is very lofty and abrupt, 
and the road girds it at a height ef 
perhaps an hundred feet from the fea, 
which 
“ 
