99 
NATURE 
[May 27, 1886 
the objective when used as a telescope; and the appearance of 
the image not only at the focus, but on each side of. it, conveys 
to the practised eye all the information required for the detection 
of the errors. 
If an objective have but one single fault, its detection is easy ; 
but it generally happens that there are many faults superposed, 
so to speak. There may be faults of achromatism, and faults of 
figure in one or all the surfaces ; faults of adjustment, and _per- 
haps want of symmetry from some strain or flexure; and the 
skill of the artist is often severely taxed to distinguish one fault 
from the rest and localise it properly, particularly if, as is gene- 
rally the case, there be also disturbances in the atmosphere itself, 
which mask the faults in the objective, and permit of their detec- 
tion only by long and weary watching for favourable moments of 
observation. 
It would be impossible in one or a dozen of such lectures as 
this to enumerate all the various devices that are practised for 
the localisation of error:, but a few may be mentioned, some of 
which have never before been made public. 
For detection of faults of symmetry, it is usual to revolve one 
lens on another and watch the image. In this way it can gene- 
rally be ascertained whether it is in the flint or crown lens. 
With some kinds of glass the curves necessary for satisfying 
the conditions of achromatism and spherical aberration are such 
that the crown becomes an equi-convex and the flint a nearly 
plano-concave of same radius on inside curve as either side of 
the crown. This form is a most convenient one for the localisa- 
tion of surface errors in this manner. 
The lenses are first placed in juxtaposition and tested. Cer- 
tain faults of figure are detected. Now calling the surfaces 
ABCD in the order in which the rays pass through them, 
place them again together with Canada balsam or castor-oil 
between the surfaces B and C, forming what is called a cemented 
objective. If the fault be in either A or D surface, no improve- 
ment is seen ; if in B or C, the fault will be much reduced or 
modified. Now reverse the crown lens, cementing surfaces A 
and C together. If same fault still shows, it must be in either 
Bor D. If it does not show, it will be in either A or C. From 
these two experiments the fault can be localised. 
It often happens that a slight error is suspected, but its 
amount is so slight that it appears problematical whether an 
alteration would really improve matters or not. Or the observer 
may not be able to make up his mind as to the exact position 
of the zone he suspects to be too high or too low, and he fears 
to go to work and perhaps clo harm to an objective on which he 
has spent months of labour, and which is almost perfect. In 
many such cases I have wished for some means by which I could 
temporarily alter the surface and see it so altered before actually 
proceeding to abrade and perhaps spoil it. 
During my trials with the great objective of Vienna, I thought 
of a very simple expedient, which effects this without any chance 
at all of injuring the surface. If I suspect a certain zone of an 
objective is too low, and that the surface might be improved by 
lowering the rest of it, I simply pass my hand, which is always 
warmer than the glass, some six or eight times round that 
particular zone. The effect of this in raising the surface is 
immediately apparent, and is generally too much at first, but 
‘the observer at the eye end can then quietly watch the image as 
the effect goes off, and very often most useful information is thus 
obtained. The reverse operation, that of lowering any required 
part of the surface, is equally simple. I take a bottle of sul- 
phuric ether and a camel’s-hair brush, and pass the brush two 
or three times round the part to be lowered, blowing on it 
slightly at the same time; the effect is immediately perceived, 
and can always be overdone if required. 
So far then for the diagnosis. Now for the remedy. When 
the fault has been localised, the lens is again put upon the 
machine and the polisher applied as before, the stroke of the 
machine and the size of the pitch patches being so arranged as 
to produce, or tend to produce, a slightly greater action on 
those parts that have been found to be too high (as before 
described while treating of the polishing processes). 
The regulation of the stroke, excentricity, speed, and general 
action of the machine, as well as the size and proportion of the 
pitch squares, and the duration of the period during which the 
action is to be continued, are all matters the correct determina: 
tion of which depends upon the skill and experience of the 
operator, and concerning which it would be impossible to 
formulate any very definite rules. All thanks are due to the 
late Lord Rosse and Mr, Lassell, and also to Dr. de la Rue, 
for having published all particulars of the process which they 
found capable of communication; but it is a notable fact that, 
as far as it is possible to ascertain, every one who has succeeded 
in this line has done so, not by following written or communi- 
cated instructions, but by striking out a new line for himself; 
and I think Iam correct in saying that there is hardly to be 
found any case of a person attaining notable success in the art of 
figuring optical surfaces by rigidly following directions or in- 
structions given or bequeathed by others. 
There is one process of figuring which is said to be used with 
success among Continental workers. I refer to the method 
called the process of local touch. In this process those parts, 
and those parts only, which are found to be high, are acted upon 
by a small polisher. 
This action is of course much more severe ; and if only it were 
possible to know exactly what was required, it ought to be much 
quicker ; but I have found it a very dangerous process. I have 
sometimes succeeded in removing a large lump or ring in this 
way (by large I mean 3 or 4 millionths of an inch), but I have 
also and much oftener succeeded in spoiling a surface by its use. 
I look upon the method of local touch as useful in removing 
gross quantities, but for the final perfecting of the surface I 
would not think of employing it. 
In small-sized objectives the remedial process is the most 
troublesome, but in large-sized objectives the diagnosis becomes 
much the more difficult, partly on account of the rare occurrence 
of a sufficiently steady atmosphere. In working at the Vienna 
objective it often happened when the figure was nearly perfect 
that it was dangerous to carry on the polishing process for more 
than ten minutes between each trial, and we had then sometimes 
a week to wait before the atmosphere was steady enough to 
allow of an observation sufficiently critical to determine whether 
that ten minutes’ working had done harm or good. It must not 
be supposed either that the process is one in which improvement 
follows improvement step by step till all is finished. On the 
contrary, sometimes everything goes well for two or three 
weeks, and then from some unknown cause, a hard patch of 
pitch perhaps or sudden change of temperature, everything goes 
wrong. At each step, instead of improvement there is disim- 
provement, and in a few days the work of weeks or months per- 
haps is all undone. Truly any one who attempts to figure an 
objective requires to have the gift of patience highly developed. 
In view of the extraordinary difficulty in the diagnostic part 
of the process with large objectives, it is my intention to make 
provision which I hope may reduce the trouble in the working 
of the new 28-inch objective for the Royal Observatory, Green- 
wich, 
Two of the greatest difficulties we have to contend with are :_ 
(1) the want of homogeneity in the atmosphere, through which 
we have to look in trials of the objective, due to varying hygro- 
metric and thermometric states of various portions ; and (2) 
sudden changes of temperature in the polishing-room. The 
polisher must always be made of a hardness corresponding to 
the existing temperature. It takes about a day to forma polisher 
of large size, and if before the next day the temperature changes 
10° or 15°, as it often does, that polisher is useless, and a new 
one has to be made, and perhaps before it is completed another 
change of temperature occurs. To grapple with these two 
difficulties I propose to have the polishing-chamber under 
ground, and, leading from it, a long tunnel formed of highly 
glazed sewer-pipes about 350 feet long, at the end of which is 
placed an artificial star illuminated by electric light; on the 
other side of the polishing-chamber isa shorter tunnel, forming 
the tube of the telescope, terminating in a small chamber for 
eye-pieces and observer. About half-way in the long tunnel there 
will be a branch pipe connected to the air-shaft of the fan, 
which is used regularly for blowing the blacksmith’s fire, and 
through this, when desired, a current of air can be sent to 
““wash it out’’ and mix up all currents of varying temperature 
and density. It may be found necessary even to keep this going 
during observations. ’ 
By this arrangement I hope to be able to have trials whenever 
required, instead of having to wait hours and days for a fayour- 
able moment. 
Figuring of Plane Mirrors.—There is a general idea that the 
working of a plane mirror or one of very long radius is a more 
difficult operation than those of more ordinary radii. This is not 
exactly the case. There is no greater difficulty in figuring a low 
curve than adeep one, but the difficulty in the case of absolutely 
plane mirrors consists simply in the fact that in their figuring there - 
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