The materials melted.

The stirring.

of technical skill.

Nineteen trials were made for one of the lenses of the 36-inch Lick object-glass, before success was attained.

A pot made of very pure clay is heated to a high temperature, and gradually filled with a batch of the raw materials. After the batch seems to be thoroughly melted a portion of it is taken out and examined, to see if any unmelted particles of silica remain, or if there are minute air-bubbles, which have not been expelled by the heat.

Should neither of these defects be discovered, the

The furnace luted.

FIG. 65.-LUMP OF OPTICAL GLASS.

swelter in the furnace heat while

covered with clay. The stirring is continued for two or three hours, until the cooling glass resists further manipulation. The

two workmen, who executing this opera

tion, must not allow the stirrer to touch the pot; for bits of clay might be ground off and mixed with the glass.

The glass is reheated, stirred a second time and even a third time, and returned to the furnace. Every open place in the furnace is stopped up, so that no air may gain admittance, and the whole is allowed to cool for several days, that it may not crack. A rapid cooling would cause it to be shattered into small fragments.

When the cooling is finished the glass is examined,

and any defects which may be apparent are ground away, or sawed off. An imperfect spot near the center Imperfections of the disc may be sawed out, if the chunk of glass is not sawed clear

through.

The accompanying figures show the block of glass from which the crown disc of the forty-inch Yerkes telescope was obtained. Fig. 65 is the original lump.

Fig. 66 shows it

after some imperfections have been sawed off. The

cut out.

disc.

lump is now to be molded into the shape shown in Fig. A forty-inch 67. The glass is put into a mold, which is placed in a special furnace and heated very slowly.

At last the

glass softens and
adapts itself to the
shape of the mold.
The temperature is
lowered to about
1,200° Fahrenheit,
and every opening
in the furnace
stopped up; after
an exceedingly
slow and careful
cooling the ten-
Fresh imper-

sided block is removed and examined.

fections are discovered and cut away, as Figs. 68 and 69 testify. The defects may be of such a nature that

Bubbles.

Striæ.

the disc must be reheated and molded again, but if too many annealings are attempted, the glass may lose its transparency. After months of labor the original shape

less mass is re

duced to a beautiful circular disc.

A few small bubbles, or bits of grit, while they mar the appearance of a disc, have no perceptible deleterious effect in a finished lens. They

prevent the pas

sage of a certain minute quantity of light, and theoretically injure the perfection of the image of an object seen through the lens.

When a careful test is made "striæ," or veins, may be found in the in

terior of the lens ;

Internal strain.

should these be

numerous or pro

nounced the lens

must be rejected.

The glass may have passed through all these tests and yet be worthless. If the process of cooling

FIG. 69.-THE LUMP CUT DOWN STILL MORE.

was not conducted with sufficient care the glass may have solidified in a state of dangerous internal strain. To test for this the glass is laid upon a piece of dark

cloth, in some place where there is suitable light, and examined by a Nicol's prism. If a pronounced dark cross is seen in the glass, the internal strain is too great, and the glass must not be used for a telescope.

The glass-founder has now finished his part of the The optician's work, and the

disc, if sufficiently perfect, is turned over to the optician, who is to fashion its curves so accurately that the rays of light from a distant star may be converged by it to a point which can

be covered with

a spider's web.

work.

The rough grinding is done with a cast-iron tool, The grinding. similar in appearance to the one lying on the floor in the illustration (Fig. 70). If a convex surface is to be produced on the glass, the tool is hollowed out and made of the proper degree of curvature. The usual grinding material is emory, which is placed between the tool and the glass. A better material is obtained by driving a blast of air into melted iron. A cloud of minute particles of iron is blown out; being chilled by contact with the air they settle down as a very fine powder.

After the lens has been brought nearly to the proper The polishing. shape it is placed upon the machine shown in Fig. 70 to be brought to its proper form by polishing. The tool, which lies upon the lens, is similar to the former one, except that its face is composed of squares

The testing.

of pitch, instead of squares of cast-iron. The lens lies
on a table which turns slowly. The tool is moved by
two wooden rods, each of which is driven by a crank at
its further extremity; the cranks are of different lengths,
and turn at widely different rates. So complicated is
the motion that the tool never describes the same path
twice. When the surface has been brought to a brilliant
polish, the lens appears to be finished.

But the most difficult part of the process is yet to
The surface, which looks perfectly spherical, is

come.

An artificial star.

probably too high in certain regions and too low in others; these inequalities must receive attention. Α spherometer which will measure 5000 of an inch is too rude to measure them. The lens is set up on edge in a special testing room, where the temperature is not subject to sudden variations; light from a lamp shining through a small hole is sent through the lens, and impinges on a mirror, which reflects it back again through the glass to the eye of the optician. To him. the entire lens appears to be aflame with light. If it is