25 
frequently be at a loss, as I confess myself to have been, from want of clear 
ideas on the subject; and the conviction that those must be obtained before 
perfection could be arrived at, has been like a weight upon my pencil, pre¬ 
venting its exertion, making me work without confidence, or satisfaction, and 
constantly in quest of the guide I wanted. 
Lairesse proposes a method of coming at the point, by making use 
of coloured patches of silk, or paper, and placing them before the eye in 
gradual succession, giving precedence in order to those which struck the eye 
first, and so on. And his method might answer, in a certain degree; but, 
in many instances, it would leave the eye doubtful and undecided, and give 
no information to the judgment, why such and such tints should be preferred, 
or placed in such, or such situations. 
But some late philosophical experiments appear to afford a surer test, as 
far as they go, and promise, I think, a certain principle to act by . 
The experiments I allude to, are those by Dr. Herschel, for ascertaining 
the illuminating, and heating power of the rays of the sun : those on he 
power of illumination answer, in fact, to what Lairesse practised with the 
coloured patches, only they were tried in anicer manner with the microscope, 
than could bfeh the naked eye ; andj^so exactly specified as to save 
any one else tCe trouble of repeating the experiments. They deteimine the 
situation of the colours in regard^ each other; but the quantity o eac i 
must either be determined by tfegr respective heat, or strength, oi by the 
prismatic proportions; possibly they may correspond: for though Doctor 
Herschel does not specify all the experiments lurried as to heat, he says 
* I have sought assiduously from books, &c. and have received information on many different 
points from tire practice of others; yet I have never met with any entire system,or theory for the 
points ,fro P h i ^ ^ oU masters tQ0k the rambow for their guide, the 
Importunity I have had of seeing their works, has prevented Reserving how they availed /*** 
themselves of thl lessons it afforded them. They also possessed a degree of philosophical know g , 
rn.de tl«« profit — ■I"'”! ” T 
it to advantage. 
E 
