Intelligence and Miscellaneous Articles. 163 
After filling both lenses with water, the first is used in a dark 
room to obtain all the phenomena presented by the objective collec- 
tion, behind the lens, of all the luminous rays which pass through 
the same: for instance, the principal focus is determined, and the 
displacement of the same by the approach of a luminous point is 
shown; this displacement is continued until the rays leave the lens 
in parallel directions, and subsequently the objective focus becomes 
a virtual one. When the luminous point is replaced by a luminous 
object, the convex water-lens also presents the series of diminished, 
equal, and magnified inverted images, and at.sufficiently small di- 
stances of the object, the magnified uninverted virtual images. 
The concave water-lens presents in air the simple phenomena of 
a diverging lens. It serves to demonstrate that, for every distance 
of the luminous point, the rays diverge on leaving the lens, that the 
foci are in consequence ail virtual ones, and that in all cases dimi- 
nished uninverted images are alone observed. 
If the media in question are now interchanged, that is to say, if 
the lenses are filled with air and surrounded by water, the optical 
phenomena, as may be easily seen, will be all reversed. 
The convex lens will become a diverging one with a virtual prin- 
cipal focus, whose distance from the lens may be easily measured 
(according to Eisenlohr’s method) by receiving upon a white screen 
the divergent solar rays proceeding from the lens in such a manner 
that the illuminated surface has a diameter double that of the lens. 
The distance of the screen from the lens will then be equal to the 
principal focal distance*. All objects (e. g. flame of a candle) behind 
the lens now appear diminished in the same manner as we usually 
find them to be with a concave glass lens. 
The concave air-lens, however, has now under water become a 
converging lens, whose principal focal length may be found by im- 
mediately ascertaining the point of convergence of incident solar 
rays. By means of a white screen immersed in the water, the course 
of the rays before and after convergence may be clearly followed, and 
the change of position of the focus, with changing distances of the 
luminous point, easily provedt. A piece of ground glass, protected 
from contact with the water by a second glass, is better than a non- 
transparent white screen for receiving the.objective inverted images : 
for the screen can only be regarded from above, and the images upon 
it appear to be raised and distorted ; whilst the ground glass can be 
looked at from behind, whence the images appear both sharp and un- 
* I employ a similar method in catoptries in order to measure the prin- 
cipal focal distance of a convex mirror. Thus between the mirror and the 
sun, and perpendicular to the rays from the latter, is placed a screen in 
which is a circular hole equal m magnitude to the portion of the mirror 
required to be operated upon; around the hole, and concentric with it, is 
described a circle of double its diameter on the side of the screen facing 
the mirror. When the screen is so placed that the reflected rays exactly 
fill this cirele, its distance from the mirror is equal to the required distance 
of the virtual focus. 
+ If it were required to surround the luminous point with water, the 
electric light between coal-points might be used. 
M2 
