THE YOUNG 
in front, of equal radius, as shown by the 
cut. The correcting thickness should be 
thrown in the front lens. If they are set 
in contact, the magnifying powers will be 
nearly as their sum ; they may therefore 
be made of double the radius, and conse- 
quently nearly twice the diameter, which, 
of course, would lessen the practical dif- 
ficulty of working a l-50th, and enable us 
to go even beyond this power. A partial 
experiment with a ith, having this " doub- 
let " front, has proved that perfect cor- 
rection for color is the result. But in the 
form tried, the spherical aberration was 
so considerable, as to require an entire re- 
construction, for w^hich I have had no 
leisure ; and though the entire success of 
the idea is yet unproved, I venture to 
record it, in case I may never be able to 
take up this subject again, as I am of 
opinion that a very perfect object-glass 
may be made of this form. 
IMMERSION OBJECT-GLASSES. 
These combinations are under-corrected, 
and not suitable for use in any other way. 
The plan is an old one, and objectives 
were constructed on this principle by 
Amici and Boss many years ago. That 
such lenses give brighter and clearer 
definition, with the highest powers, from 
the l-12th upwards, is unquestionable. 
BRASS CELLS FOR OBJECT-GLASSES. 
For the brass setting of object-glasses, 
it is necessary that the w^orker should pos- 
sess a good foot-lathe ; if provided with a 
self-acting arrangement for chasing up 
the short screwed parts of the cells, this 
will insure greater accuracy of workman- 
ship. The setting or metal work of an 
object-glass must always be made before 
the lenses are commenced ; three steel 
gauges 'are to be first formed, of a width 
exactly corresponding to the diameter of 
the intended lenses; this gauge I make 
out of a piece of sheet steel, with three 
arms of the three diameters required. A 
chuck should be fitted to the lathe, and 
cut out to the standard thread now gener- 
ally adopted for object-glasses ; into this 
the brass setting is fitted, and each cell 
screwed on, and turned out in succession 
to the proper size. I leave no shoulders 
SCIENTIST. 107 
at the back of the cells, but bore them 
clear through. 
Triblet tubing is not sufficiently accu- 
rate for the outer shell of the highest 
powers; it is better, therefore, to make 
this of one casting, and bore it out of the 
solid, from its own chuck, and finish to 
the size with a fluted rimer. I have always 
made the inner tube, containing the back 
lenses, to traverse to and fro, in preference- 
to the front lens, as the object is not 
thereby lost sight of during the adjust- 
ment, which is performed in one-third of 
a revolution of the outer ring, which has. 
an inclined groove cut in it, acting on a. 
screwed pin connected with the inner tube. 
This plan is more simple in construction, 
and less liable to derangement than the^ 
one commonly employed. 
Our American Geysers. 
THE two most wonderful geysers in 
the world, those known as " The 
Giantess" and "Great Geyser," are 
located in our National Park, on the Yel- 
lowstone Kiver. Here, within an area of 
sixty miles square, in the north-west cor- 
ner of "Wyoming Territory, are collected 
such a variety of natural wonders, that, 
when the ways of approach are open, 
Americans will not need to leave their 
own country in order to view the grandest 
of all natural phenomena. Through one 
portion of this park flows the Fire Hole 
Kiver ; and it is in that part of its valley 
known as the Fire Hole Basin, that these 
Geysers are found. Here within an area 
of about five square miles may be found 
from twelve to fifteen hundred springs, 
with basins of all sizes, from a few inches 
in diameter to three hundred feet. These 
springs are of three kinds: 1. Those in 
which the ebullition occurs only at inter- 
vals, which are the true geysers ; 2. Those 
which are constantly boiling; and, 3. 
Those which are in a state of repose. 
The general appearance of these geysers 
is well shown in the illustrations on next 
page, though their grandeur and mag- 
nificence can never be even faintly appre- 
ciated unless they are seen in action. An 
eye-witness thus describes them : 
On a search for new wonders, leading 
