146 
DE. T. E, EOBINSON AND ME. T. GEUBB’S DESCEIPTION OF 
in that third marked A its primary lever, in that marked B its two secondaries, and in 
that of C its six tertiary, in which the four triangular are triple and the two straight are 
double. These tertiary levers support the speculum by intervening balls, as originally 
adopted by Lord Rosse ; but it has not been attempted to keep these balls in their place 
by a wire and spring, which has been found occasionally to fail, but by being surrounded 
by a metallic ring allowing of a slight rolling movement, but affording no chance of 
displacement. The balls, 1^ inch diameter, are of hard cast iron, made approximately 
true by grinding. 
In Plate Y. fig. 10, 1 is a vertical section of a primary lever, 2 a secondary, and 3 a 
tertiary with its balls and rings. Figs. 13 & 14, taken from photographs, further illustrate 
the system of vertical support ; fig. 13 shows the supports as described ; fig. 14 the same, 
but with the addition of two rings of sheet iron which are necessary for keeping the 
levers in their proper position laterally. 
The lateral support of the specula is that originally described in a paper by Mr. Grubb 
already referred to. It is shown in section in Plate Y. fig. 10, where D is a ring of 
wrought iron trued after being attached to the inner side of the speculum-box, and E 
is apother ring of same material which fits with a slight looseness on D, and also fits 
loosely the trued edge of the speculum. It may be seen that, except when the speculum 
is quite horizontal, its weight pressing against the lowest part of the ring E must bring 
its opposite or highest part into close contact with the ring D, while that part of the 
ring E, which the speculum is pro tempore bearing against, is thereby removed out of 
contact with the ring D, and consequently free to move up and down a small but suffi- 
cient quantity, which virtually places the speculum under equal advantages as respects 
lateral supports as if it were held in a flexible band, of the form of the letter U, supported 
at its upper ends. 
This method of lateral support was applied long since by Mr. Grubb to smaller spe- 
cula, including one of 20 inches diameter at the Glasgow Observatory, and with entire 
success ; but as grave doubts were expressed recently as to its efficacy in the case of a 
4-feet speculum, it is desirable to mention that its efficiency is as decided in this latter 
case as in that of the 20-inch ; no appearance of flexure being perceptible in either 
instance on placing the telescope in various positions, by which different parts of the 
speculum, including opposite sides of same, were brought successively into the lowest 
position.] 
The tube consists of two parts ; the lower, which is made of strong sheet iron, is firmly 
secured above in the cradle of the declination-axis, and below to the speculum-box by 
three strong holts. Between two of these is a slit, through which, by means of guides 
passing through two openings above it, a cover which fits closely in the ring C (Plate Y. 
fig. 10) can be introduced to protect the speculum when not in use. Then also the 
large eyepiece is replaced by a vessel containing quicklime to absorb moisture. This 
part of the tube is T feet long, of which only 17 inches are below the cradle ; so the 
speculum is not quite 40 inches below the centre of the declination-axis. The upper 
