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the distance of seventy feet, with forty plain mirrors.

James. I do not see how they can act as burning glasses.

Tutor. A plain mirror reflects the light and heat coming from the sun, and will illuminate and heat


substance on which they are thrown, in the same manner as if the sun shone upon it. Two mirrors will reflect on it a double quantity of heat; and if 40 or 100 mirrors could be so placed as to reflect from each the heat coming from the sun, on any particular substance, they would increase the heat 40 or 100 times.

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Of Concave Mirrors-their Uses

how they act.

JAMES. To what uses are concave mirrors applied ?

Tutor. They are chiefly used in reflecting telescopes; that is, in telescopes adapted to viewing the heavenly bodies. And as you like to look at Jupiter's little moons and Saturn's ring through my telescope, it may be worth your while to take some pains to know by what means this pleasure is afforded you.

Charles. I shall not object to any attention necessary to comprehend the principles on which these instruments are formed.

Tutor. A B (Plate 1. Fig. 16.) represents a concave mirror, and a b, c d, ef, three parallel 'rays of light falling upon it.

c is the centre of concavity, that is, one 'leg of your compasses being placed on c, and then open them to the length cd, and the other leg will touch the mirror-a B in all its parts.

James. Then all the lines drawn from c to the glass will be 'equal to one another, as o b, c d, and of?.

Tritor. They will: and there is another property belonging to them; they are all perpendicular to the glass in the parts where they touch.


Charles. That is c b, and cf are perpendicular to the glass at b and f, as well as c d at d.

Tutor. Yes, they are:-cd is an incident ray, but as it passes through the centre of concavity, it will be res flected back in the same line, that is, as it makes no angle of incidence, so there will be no angle of reflection: a b is an incident ray, and I want to know what will be the direction of the reflected ray?

Charles. Since c b is perpendicular to the glass at b, the angle of incidence is a b c; and as the angle of reflection is always equal to the angle of incidence, I must make another angle, as cbm, equal to abc*,


* To make an angle c b m, equal to another given one, as ab c. From 6, as a centre with


and then the line b m is that in which the incident ray will move after reflection.

Tutor. Can you, James, tell me how to find the line in which the incident ray e f will move after reflection?

James. Yes: I will make the angle c fm equal to cf e, and the line:f m will be that in which the reflected ray will move; therefore e f is reflected to the same point' m as a b


Tutor. If, instead of two incident rays, any number were drawn paral


any radius 6 r, describe the arc x 0, which will cut ċ b in z, take the distance x z in your compasses, and set off with it z o, and then draw the line bom, and the angle mbci equal to the angle a b c.

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