determined that nothing shall escape your Notice which is deferving of it, and is, at the fame Time, in my Power to lay before you; and therefore I fhall hereafter fupply you with other Ways and Means for viewing fmall Objects:I mean, by those Inftruments we may properly call catoptric Microscopes, or fuch where the Vifion of Objects is effected by reflected Light; and you must know, and I doubt not but you will then find, by Experience, that this is the most exquifite and perfect Kind of Vifion that Nature affords: For all the Microscopes that we have hitherto been contemplating the Nature of, confift of Lenfes only, and produce their Effects by refracted Light; therefore the Vifion of an Object will be imperfect upon two Accounts, the firft is, because the regular Refraction of Rays will be obftructed by the Figure of the Glafs; because the Rays which go through one Part of the Glafs are not refracted precisely to the fame Point with those which pafs through another Part; and therefore every individual Point in the Object cannot be represented by a fingle Point in the Image, but will be, as it were, dilated into a small Space, and, confequently, the feveral Points of the Object will be confufedly blended together in the Image; therefore it must not be viewed by a Glafs that magnifies too much, left it should discover the Imperfection of the Image, and render the View difagreeable.

Euphrof. You are now upon fuch a nice Subject, and what I have beeh fo little used to think of, that I can but just comprehend your Meaning.-I think, you intend I fhould underftand, that unless every Point of the Object, which fends forth Rays to the Glass, could have all those Rays collected into a fingle Point, the Vifion or Image of that Point cannot be diftinct and perfect.-Am I right, Cleonicus?

Cleon. It is the very Idea I endeavoured to convey ; but that is not to be abfolutely effected by Glaffes :Befides this, there is another, and ftill greater Caufe of imperfect Vifion by refracted Light, and that is, the different Refrangibility of the Rays of common Light, that is to fay, the Rays of Light, proceeding from the fame Point of an Object to the fame Part of the Lens, will fome of them be refracted to one Point in the Axis,

and

INGLE, COMPOUND,and SOLAR MICROSCOPES.

and fome to another: Or, in other Words, fome will be more, and others lefs refracted, and, confequently, the several Points of the Object will be very much dilated and confused, in the fame Manner as I faid before was occafioned by the Figure of the Glafs; but in a much higher Degree:- -Nor is this the only Misfortune of refracted Vision arifing from the Rays being differently refrangible; for each particular Sort of Rays that are contained in a Beam of common Light, will, at the fame Time, act differently upon the optic Nerve, and produce different Ideas of Colour, according to its different Degree of Refrangibility; and therefore one and the fame Point of an Object is not only, by Refraction, multiplied, as it were, into many Points; but is likewife diverfely coloured in each of thofe Points in the Image, and, confequently, when the Image of any Object, formed by thofe Glaffes, are viewed by a very deep Magnifier, they will appear not only very indiftinct and diftorted, but variously coloured at the fame Time.Upon all which Accounts it is eafy to obferve, that we can arrive at greater Powers of magnifying fmall Objects by fingle Microscopes, than by compound Ones; becaufe, in the former, we view the Object itself, and in the latter its imperfect Image :- But as one of those Caufes of the Imperfection of Vifion is much lefs by reflected Light, and the other wholly avoided, it will, from thence, appear how much preferable a Catoptric Microscope is to a Dioptric One.-But, that you may be apprized more thoroughly of this important Subject, which is the Foundation of the Theory, or Doctrine of Colours; I must take another Opportunity of inftructing you therein, by fuch Experiments as I make no doubt will afford you a very agreeable Entertainment, as well as give you a more accurate Infight into the Nature of Vision, and the Perfection of optical Inftruments; efpecially that Part we call Telescopes, the Nature of which cannot be well underflood without it.

DIALOGUÈ V,

Of the COLOURS of natural BODIES, and of the RAIN-BOWS, illuftrated by Experiments of the PRISM, &C.

Euphrofyne.

THEN we laft converfed together, you raised my

W Expectation in regard to the Theory of Colours,

I think, you then told me, that the different Refrangibility of Rays was the Caufe of Colours in all the various Objects we view, and that the Proof of this was easy for me to understand, by Experiments: If this be fo, Cleonicus, nothing will equal the Pleasure and Happiness which the prefent Hour will afford me.

Cleon. You will find every Thing I have faid, relative to this Subject, to be true.As the Pride, Gaiety, and Beauty of Nature appears more, in the rich Variety of Colours, than in any one Thing befides, and feem as if intended in a particular Manner to pleasure and adorn your Sex; I know of nothing that can prove more a propos for a Lady's Study than the beautiful Doctrine of the Colours of Light, and which is more eafily attainable by Experiments of the Prism.

Euphrof. I have partly experienced already the Truth of what you now fay.- You remember very well,

when we were Children, how often we diverted ourselves with the beautiful Colours that we obferved in the Prifm, that my Father had in his Study, and how often we delighted ourselves with viewing the exquifite Circles of coloured Light, which we always obferved in thofe thin Bubbles which we blowed up out of Soap-water with the Shaft of a Tobacco-pipe.I little thought then that these were Subjects of fo curious and philofophical a Nature, as I now find they are.-The wonderful Colours of the celestial Bow, in like Manner, ftrike every one's Eye; but the Phænomena of Colours, beautiful as they are, are more generally the Subject of Senje, than of the Understanding.