Bovxin—The Automatic Image- Finder. 287 
length is fixed; and at the intersection of the rays a screen is 
placed to catch the image of the candle. Now, by moving the 
arrow in order to catch the image, the screen must be placed at 
the intersection of the rays, and nowhere else, thus proving experi- 
mentally the truth of the above principles, and the correctness of 
the working of the instrument. 
4°,.— Explanation of the Construction of Optical Instruments by means 
| of the Image-Finder. 
Lastly, the Automatic Image-Finder enables us to explain 
with ease the construction of the various optical instruments. All 
we have to do is to state precisely what we desire to get, and then 
apply our principles and our instrument to secure it. 
For example, suppose it is required to construct a projection 
apparatus, what is wanted is an enlarged real image. Now, to 
secure this, the object must be placed outside the principal focus, 
else a virtual image would result; and since the image is to be 
larger than the object, it is found by moving the arrow that it 
must be placed between f and 2/7. 
In a word, we see that a projection apparatus consists of a 
double convex lens of moderate focal length, the object being 
placed between f and 27. 
Again, if it is required to find out what the construction of 
the microscope is, there must be a virtual image, and it must 
be larger than the object. This granted, it follows at once 
that it must consist of a convex lens, for no other lens. gives an 
image larger than the object (in a double concave lens the image is 
always smaller than the object). Now since the image is to be 
virtual and the lens convex, the object must be placed inside the 
principal focus and only a little inside, else the image is not as 
large as possible. Al this is clearly seen by moving the arrow in 
the instrument into various positions. 
The telescope, camera, &c., admit of a similar easy explanation 
by means of this instrument. 
