382 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
in the figure the principal focus of the glass sphere is about 8 instead 
of one and a half radii, as it ought to be. An instrument constructed 
on the proportions as shown in this figure would therefore be useless. 
This is a strange mistake, because the focus of a sphere is correctly 
given in another part of the book. 
It is intended that at the hours the sun’s rays should burn a string 
to which a bell is attached, thus causing it to ring. 
We now come to the most interesting figures, viz. 70 and 71. These 
are called “ Catoptrico dioptrica telescopica,” and are two out of a series 
Fig. 70. 
of seven. In fig. 70, D C is the object-glass of a telescope, and A B its 
eye-lens ; by means of the two mirrors H G and E F the telescope is, as 
it were, folded up into five. The arrow at X is intended to represent an 
erect image. It will be noticed that there are two reflections at each 
mirror. This is the second in the series ; the first has not been copied, 
as the arrangement can easily be understood from fig. 70 ; the difference 
between them being that there is only one reflection at each mirror in- 
stead of tw T o, so that the telescope is folded into three instead of into 
five. This leads to fig. 71, which is the third of the series, and shows an 
arrangement of plane mirrors, by which the path of the rays is diverted 
Fig. 71. 
in precisely the same manner as by Porro’s prisms. This is the prin- 
ciple which underlies the Zeiss patent binoculars. It will be noticed 
that the eye-lenses in figs. 70 and 71 are plano-convex, and are turned in 
their proper positions. 
The seventh and last of the series has four plane mirrors, and is 
very ingenious. 
The axis of the telescope is supposed to be horizontal ; the rays are 
intercepted just behind the object-glass by a plane mirror placed at an 
angle of 45°, and are reflected vertically downwards on to the second 
plane mirror, placed nearly horizontally. This reflects the rays verti- 
