The Description Of The Glasses Continued

Then the diameter of this glass does not need to be so large as for the preceding bezel, nor must it also be so small as that of the glass A[70] of the other before, but it must at approximately be such that the straight line NP passes through the inner hot point of the hyperbola NRP; for, being less, it would receive fewer rays from the object Z, and being larger, it would receive very little more; so that its thickness, having to be in much greater proportion than before, it would deprive them of their strength as much as its size would give them; and, besides that, the object could not be so illuminated.

It will also be good to put this telescope on some machine like ST, which holds it directly turned towards the sun. And you have to encase the NOPR glass in the middle of a hollow parabolic mirror like CC, which collects all the rays of the sun at the point Z on the object which must be supported there by the small arm G, which comes out of some place of this mirror: and this arm must also support around this object some black and obscure body, like HH, precisely of the size of the NOPR glass, so that it prevents that none of the rays of the sun fall directly on this glass; for from there, entering the pipe, some of them could be reflected towards the eye and weaken the vision accordingly; even though this tube must be completely black on the inside, it cannot be so perfectly so that its material does not always cause some reflection when the light is very bright, as is that of the sun.

Besides this, this black body HH must have a hole in the middle, marked Z, which is of the size of the object, so that, if this object is in any way transparent, it can also be illuminated by the rays which come directly from the sun; or even again, if need be, by these rays picked up at point Z by a burning glass, like II, of the size of NOPR glass, so that there comes from all sides as much light on the object as there is. can suffer without being consumed by it, and it will be easy to cover a part of this mirror CC or of this glass II, to prevent too much from coming into it. You can see why I take so much care here to ensure that the object is very well lit, and that many of these rays come towards the eye; because the NOPR glass, which in this telescope performs the office of the pupil, and in which those of these rays which come from various points intersect, being much closer to the object than to the eye, is the cause that they extend over the extremities of the optic nerve in a space much larger than is the surface of the object from which they come; and you know that they must have so much less force there as they are more extended there, as one sees, on the contrary, that being gathered in a smaller space by a mirror or burning glass, they have more. : and it is from this that the length of this telescope depends, that is to say the distance which must be between the hyperbola NOP and its burning point; for the longer it is, the more extensive the image of the object is in the fundus of the eye, which causes all its small parts to be more distinct there: but this itself also weakens so much their action that finally it could no longer be felt if this telescope were too long; so that its greatest length can only be determined by experience, and even it varies according as the objects can have more or less light without being consumed by it. I know very well that some other means could still be added to make this light stronger; but, apart from the fact that they would be more difficult to put into practice, we would hardly find objects that could suffer more from them. One might well also, instead of the hyperbolic glass NOPR, find others which would receive somewhat greater quantity of rays, but where they would not cause these rays, coming from various points of the object, to assemble so exactly towards the eye at as many other different points, or it would be necessary to employ two glasses there instead of one, so that the force of these rays would not be less diminished by the multitude of surfaces of these glasses which it would be augmented by their figures, and finally the execution would be much more difficult. Only do I still want to warn you that these glasses can only be applied to one eye, it will be better to bandage the other or cover it with some very dark veil, so that its pupils remain as open as possible. may only leave it exposed to the light or close it with the help of the muscles which move his eyelids; for there is usually such a connection between the two eyes that one can hardly move in any way without the other preparing to imitate it. Moreover, it will not be useless not only to press this telescope close to the eye, so that no light can come towards it except through it, but also to have softened his sight beforehand by holding himself in dark place, and to have the imagination disposed as if to look at very distant and very dark things, so that the pupil opens all the more, and so that we can see an object all the larger. Because you know that this action of the pupil does not follow immediately from the will that one has to open it, but rather from the idea or the feeling that one has of the darkness and the distance of the things that we look.

Besides, if you reflect a little on all that has been said above, and particularly on what we have required from the external organs to make the vision as perfect as it can be, there is no You will not find it difficult to hear that, by these various forms of spectacles, one adds to them all that art can add to them, without it being necessary for me to stop to deduce the proof of it for you at greater length. It will not be difficult for you either to know that all those we have had up to now could not in any way be perfect, since there is a very great difference between the circular line and the hyperbola, and that we has only tried by making them use this one for the effects for which I have demonstrated that this one was required; so that we have never been able to find that when we have failed so fortunately, that, thinking of making spherical the surfaces of the glasses that we have cut, we have made them hyperbolic, or of some other equivalent figure. And this principally prevented that one could not make well the glasses which are used to see the inaccessible objects, because their convex glass must be larger than that of the others; and, besides the fact that it is less easy to find a lot than a little, the difference which is between the hyperbolic figure and the spherical one is much more noticeable towards the extremities of the glass than towards its centre. But, because the craftsmen will perhaps judge that it is very difficult to cut the glasses exactly according to this hyperbolic figure, I will still try here to give them an invention by means of which I am convinced that they can quite easily overcome.