Page images

picture be removed to a greater distance ? Tutor. It will : for then the

rays will fall more diverging upon the glass, and cannot be so soon collected into the corresponding points behind it. Charles. From what

you have said, I see that if the object A B C be placed in F, the rays, after refraction, will go out parallel to one another; and if brought nearer to the glass than F, then they will diverge from one another, so that in neither case an image will be formed behind the lens.

James. To get an image, must the object be beyond the focus F?

Tutor. It must: and the picture will be bigger or less than the object,

as its distance from the glass is greater or less than the distance of the object; if A B C (Fig. 12.) be the object, e b A will be the picture; and if c b a be the object, A B C will be the picture.

Charles. Is there any rule to find the distance of the picture from the glass? Tutor. If


know the focal distance of the glass, and the distance of the object from the glass, the rule is this:

Multiply the distance of the focus, by the distance of the object, and divide the product by their difference, the quotient is the distance of the picture.”

James. If the focal distance of the glass be seven inches, and the

object be nine inches from the lens,

7X9 63 I say,

= 311 inches; of 2 course the picture will be very much larger than the object. For, as you have said, the picture is as much bigger or less than the object, as its distance from the glass is greater or less than the distance of the object.

Tutor. If the focus be seven inches, and the object at the di. stance of seventeen inches, then the distance of the picture will be found

7X17 119 thus

12 inches 10

10 nearly.


Images of Objects invertedOf the

Scioptric Ball Of Lenses and their Foci.

JAMES. Will the image of a candle, when received through a convex lens, be inverted?

Tutor. It will, as you shall seer: Here is no light in this room but from the candle, the rays of which pass through a convex lens, and by holding a sheet of paper in a proper place, you will see a complete inverted image of the candle on it.

An object seen through a very small apertựre appears also inverted, but it is very imperfect compared to an image formed with the lens; it is faint for want of light, and it is confused because the


interfere with one another.

Charles. What is the reason of its being inverted ?

Tutor. Because the rays from the extreme parts of the object must cross at the hole. If you look through a very small hole at any object, the object appears magnified. Make a pin-hole in a sheet of brown paper, and look through it at the small print of this book.

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