SrSir

I herewith send you an answer to the Iesuite Pardies considerations,Ph. Tr. N. 83. p. 4059 in the conclusion of wchwhich you may possibly apprehend me a little too positive, but I speake onely for my selfe. I am highly sensible of yoryour good will in communicating to me such observations as occur concerning my Theories or Catadioptricall instruments, & I desire you to continue that favour to me. I shall immediately proceed to add what I promised to my answer to Mr Hooks Observations, & then send it you. MonsrMonsieur Hugenius has very observed the confusion of refractions neare the edges of a Lens where its two superficies are inclined, much after the same manner as like the planes of Prism whose refractions are in like manner confused. But it is not from yethe inclination of those superficies so much as from yethe htereogeneity of light that that confusion is caused. For by illuminating an object wthwith homogeneall light, I have seen it far distincter through a Prism then I could by light that was heterogeneall.

I suppose the designe of SrSir RtRobert Morays Experiments is to have their events expressed, wthwith such observations as may occur concerning them.

1. Touching the first I have observed that the solar image falling on a paper placed at yethe focus of the Lens, was by the interposed Prism drawn out in length proportionall to the Prisms refraction or distance from that focus. And yethe cheife observable here wchwhich I remember was that the streight edges of the oblong image were distincter then they would have been without the lens.

Considering that the rays comeing from the Planet Venus are much lesse inclined one to another then those wchwhich come from the opposite parts of the suns disque, I once tryed an experiment or two wthwith her light. And to make it sufficiently strong I found it necessary to collect it first by a broade lens, & then interposing a Prism between the Lens & its focus at such distance that all the light might passe through the Prism, I found the focus wchwhich before appeared like a lucid point, to be drawn out into a long splendid line by the Prisms refraction.

I have sometimes designed to try how a fixt starr seen through a long Telescope would appeare by interposing a Prism between the Telescope & my eye. But by the appearance of Venus viewed wthwith my naked eye through a Prism I presage the event.

2. Concerning yethe second Experiment I have occasionally observed that by covering both ends of the Prism wthwith paper at severall distances from the middle, the breadth of the solar image will be increadsed or diminished as much as is the aperture of the Prism wthoutwithout any variation of the length. Or if so much & no more augmented.

3. Of the third experiment I have occasion to speake in my answer to Mr Hook, where you will find the effects of two Prisms in all crosse positions of one to another described. But if one Prism alone be turned about, the coloured image will onely be translated from place to places, describing a circle or some other conick section on the wall on wchwhich it is projected, wthoutwithout suffering any alteration in its shape, unlesse such as may arrise from the obliquity of the wall, or casuall change of the Prisms obliquity to yethe sun's rays.

4. The effect of the fourth Experiment I have already insinuated telling you (in pag 3076 of the said Transactions) that light passing through the parts of the Prism of divers thickcnesses did still exhibit the same Phænomena.

ITm The long axes of the two Prisms in the experiment described in the said pag 3076 of the Transactions were parallel one to another. And for the rest of their position you will best apprehend it by this scheme, where let EG designe the window, F the hole in it through wchwhich the light arrives at the Prisms, ABC the first Prism wchwhich refracts the light towards PT, painting there the colours in an oblong forme, & αβγ the second Prism wchwhich refracts back againe the rays to Q, where the long image PT is contracted into a round one. The plane αγ to BC, & βγ to AC I suppose parallell, that the rays may be equally refracted contrary ways in both Prisms,. And the Prisms must be placed very neare to one another. For if their deistance be so great that colours begin to appeare in the light before its incidence on the second Prism, those colours will not be destroyed by the contrary refractions of that Prism.