THE YOUNG SCIENTIST. 
119 
When collecting on the Massachusetts 
coast, I often visited the fishing smacks, 
for the purpose of examining the contents 
of the stomachs of various kinds of 
fishes. I always found them packed solid 
with food. Why do fish go to all the 
trouble of eating the different kinds of 
fish if it is not for some good purpose- 
to sustain life. Why have they teeth if 
they are not of any use ? 
When at Barnum's, the fish were fed at 
three o'clock ; at that time they became 
uneasy, and at the first stroke of the 
chopping knife on the feeding board, every 
fish was to the front of his tank, often 
pressing their noses against the glass. 
The crabs would become frantic and 
scramble all over the rockwork, searching 
for their food. Our greatest anxiety with 
new fish was to get them to feed, then we 
considered they were out of danger. 
Alfred Lloyd, of the Crystal Palace 
Aquarium, London, who, I consider, is 
the very best authority on aquaria, feeds 
to the animals contained in the tanks 
thousands of pounds of food yearly, 
consisting of thirteen kinds — both animal 
and vegetable. 
Home-made Telescopes and Micro- 
scopes.— X. 
NEW FORMULA FOR A MICROSCOPE OBJECT- 
GLASS BY MR. WENIIAM. 
A PENCIL of rays exceeding an angle of 
40° from a luminous point cannot be 
secured with less than three superposed 
lenses of increasing focus and diameter, 
by the use of which combination rays be- 
yond this angle are transmitted, with suc- 
cessive refractions in their course, towards 
the posterior conjugate focus. Until quite 
recently, each of these separate lenses 
has been partly achromatized by its own 
concave lens of flint-glass, the surface in 
contact with the crown-glass being of the 
same radius, united with Canada balsam ; 
the front lens has been made a triple, the 
middle a double, and the back again a 
triple achromatic. This combination, 
therefore, consists of eight lenses, and the 
rays in their passage are subject to errors 
arising from sixteen surfaces of glass. 
In the new form there are but ten sur- 
faces, and one concave lens of dense flint 
is used for correcting four convex lenses 
of crowm-glass. As this might at first sight 
be considered inconsistent with theory, 
a brief retrospect of the early improve- 
I ments of the microscope object-glass will 
! help to define the conditions. The know- 
• ledge of its construction has been entirely 
iin the hands of working opticians; 
land the information published on the 
' subject being scanty, this has prob- 
^ably prevented the scientific analyst 
I from giving that aid which might have 
I been expected. 
j Previous to the year 1829 a few micro- 
scopic object-glasses were made, comi^osed 
I of three superposed achromatic lenses ; 
but this combination appears to have 
been used merely with the intention of 
gaining an increase of power in ignorance 
of any principle, and without even a 
knowledge of the value of angular aper- 
ture. 
At this time the late J. J. Lister tried a 
number of experiments, and discovered 
the law of the aplanatic focus, and 
[ proved that, by separating lenses suitably 
i corrected, there were one or two positions 
I in wdiich the spherical aberration was 
I balanced. This was explained in a paper 
1 read before the Koyal Society in 1829. In 
the year 1831 Mr. Koss was employed to 
j construct the first achromatic object-glass 
I in accordance with this principle, which 
j performed "with a degree of success 
never anticipated." 
Mr. Eoss then discovered that, after he 
had adjusted the interval of his lenses for 
the aplanatic focus, that position would 
no longer be correct if a plate of thin 
glass was placed above the object; this 
focus had then to be sought in a different 
plane, and the lenses brought closer to- 
gether, in order to neutralise the negative 
aberration caused by covering-glass of 
various thickness. From this period the 
"adjustment" with which all our best 
object-glasses are now provided, became 
established. Fig. 14 is the form of object- 
glass used at this time, consisting of three 
plano-concave achromatics, v^hose foci 
w^ere nearly in the proportion of 1, 2, 3. 
No greater angle than 60° could be ob- 
