Light, appeared conftantly of the Colour of those Rays that fell upon it, because those Rays were alone reflected to the Eye; and, therefore, in regard to any Colours in Objects by Reflection or Refraction, I am taught, by these Experiments, to understand, that they proceed only from fuch or fuch particular Sort of Rays, as are reflected or refracted to the Eye.

Cleon. You apprehend this Affair very happily; the Thing you mention comprehends near the whole Rationale of the Doctrine of Colours-when Objects reflect or refract the Rays fingly, they must neceffarily appear of the Colour proper to thofe Rays ;- -but fome Sorts of Matter will reflect one Sort of Rays and refract another; and therefore appear of one Colour by Reflection, and another by Refraction; thus, for Inftance, Leaf-gold reflects the yellow-making Rays, and therefore YELLOW is its common Colour, but it tranfmits the green and the blue Rays, and therefore appears of an azure Colour, by refracted Light.- Such Bodies as throw up all the Rays promifcuously muft appear white, which is the natural Colour of the Sun's Light, where all the different coloured Rays are uniformly blended together: On the other Hand, fuch Bodies as imbibe all the Light, and reflect little or none, appear of a dark, or black Colour, if I may fo fpeak, for in Strictnefs of Speech, Blackness is the Abfence of all Colour, rather than a Colour itself:- -Laftly, as two or more different coloured Rays are reflected and refracted from Bodies they will exhibit a compound or motley Colour, and thus all Variety of Colours in Bodies may be readily accounted for by the different Refrangibility and Reflexibility of Light.But I muft recommend to your Perufal fome eafy Treatife on this Subject, which will farther enlarge your Ideas and Notion of the Doctrine of Light and Colours; the principal of which is Sir Ifaac Newton's Treatife of OPTICS.*

• The Reader may also please to perufe our Inftitutions of DIOPTRICS, especially the popular Part; alfo Chap. VII. on the Subject of prifmatic Colours, p. 140.-And farther, for

Euphrof. Well, I have pretty well tired you for this Time; I fhall perufe that great Author's Book, and shall not be a little proud if there be any Thing in his Writings that can pretend to understand. - Pray, Cleonicus, what is to be the Subject of our next philofophical Con ference?

Cleon. By the prefent Experiments and Difcourfe, I have endeavoured to prepare you for understanding the Doctrine and Rationale of Telescopes, the different Forms, Conftruction, and Ufes of which I fhall difcourfe of to you as Opportunity will permit, and which, I make no Doubt, will prove Subjects of agrecable Entertainment.

On the Original of TELESCOPES; and their general NATURE, KINDS, and USES explained.

WEL

Euphrofyne.

ELL, Cleonicus, here is an illuftrious Appearance of various Kinds of Inftruments, most certainly; pray, are they all of that Kind you call Telefcopes?

Cleon. They are, my Euphrofyne, but of different Sorts and Forms.

Euphrof. Pray, what is properly imported by the Word Telescope? for I obferve it is a general Name for Inftruments of many different Sorts.

Clean. The Word Telescope is of Greek Original, and fignifies an Inftrument intended to perfect the Sight, that is to fay, to improve and perfect the Sight of any Object,

which

Illuftration of the foregoing Doctrine of the Rain-bow, we have added a Copper-plate (Numb. XLVI.) to render vifible to the Eye the Manner in which the Beam of Light enters the Drop, is reflected in it, and refracted from it, in the Two Experiments for fhewing the Colours of each Bow, respectively; and which, we fuppofe, can need no other Explanation than bare Infpection.

which the Eye alone is not fitted to difcern; these I have obferved to be fuch as are exceeding fmall, and others which are very remote; for the Limits of natural Vision, are, on one Hand, the Distance of about fix or seven Inches from the Eye; and, on the other, an Angle of about one Minute of a Degree; unless diftant Objects are feen under, an Angle greater than that, they cannot be diftinctly viewed by the Eye; and though the Word Telescope is general, and comprehends all Glaffes that tend to improve Vifion, yet Cuftom has reftrained it to mean that particular Sort only, which relates to diftant Objects; fo that, by a TELESCOPE, is ufually fignified an Inftrument that perfects the View of diftant Objects.

Euphrof. Is the Invention of Telescopes of antient or modern Date?

Cleon. It is certain that it is not of very antient Date; the famous Roger Bacon, before the Year 1300, was the first that we find making any particular Mention of Inftruments for magnifying fmall Objects, or making very remote Objects appear plainly to the View; and it is worth while for you to be acquainted with fome of his Expreffions, to this Purpofe, which are as follow;

for thus Lenfes and Speculums may be figured, that one Object may be multiplied into many, -that thofe which are pofited at a great Distance may be made to appear very near; that thafe which are fmall may be made to appear very large, and thofe which are obfcure to appear very plain; -and we can make Stars to appear where we will. -One would think, from fuch kind of Expreffions as these, and other Tracts which he wrote upon the Subject of Vifion, that, this, Author muft have been well acquainted with the Nature, Conftruction, and Ufe of Telescopes, and all the Glaffes which compofe them. But our modern Critics in Optics, deny him, not only the Invention, but even the Knowledge of any fuch Conftruction as we at prefent call Telescopes.

Euphrof. I cannot fay but I fhould at once have concluded that Roger Bacon, by his Manner of fpeaking, had been an excellent Optician; but if he was not, can you tell me, Cleonicus, who was the Author of fo useful an Invention?