DR. OLIVER LODGE ON ABERRATION PROBLEMS. 
763 
Operation. 
Effect on vertical bands. 
Effect on horizontal bands. 
Pressing lightly and intermit¬ 
tently downward.s on corner of 
large wooden frame. 
Concertina action ; no shift of 
middle band. 
Either rotation of whole set 
about a point, or shift up and 
down of whole set, or mere 
blurring. 
Turning slightly one of the sup¬ 
porting screws of front or 
“ fourth ” mirror. 
Concertina action and tilting of 
all bands, except the middle 
one ; no shift of middle one 
Shifting of entire set, and dis¬ 
turbance of bands. 
Vertical screw supporting semi¬ 
transparent plate turned so as 
to tilt its plane about a hori¬ 
zontal axis, the axle being two 
inches to one side of the plate. 
No concertina action, but a rapid 
blurring of the bands. 
Concertina action, and tilting of 
all bands but the middle one ; 
no shift of middle one. 
Horizontal screws supporting 
semi-transparent plate turned 
so as to tilt it about a vertical 
axis, the axle being near one 
end of the plate. 
Concertina action; no shift of 
middle band. 
No concertina action, but a 
rapid blurring of the bands. 
The effects of vertical and horizontal screws were thus complementary on the two 
sets, as M^as natural. 
37. In order that the dark bands shall be really dark, a nice adjustment of the thick¬ 
ness of silver film on the semi-transparent plate is necessary. It can only be hit on by 
a sort of chance, for when once taken out of the silvering bath it is useless to put it 
back again if not sufficiently done. 
The equality of transmission and reflexion at 45° is readily tested by two gas flames 
a yard or two apart, with the plate at the corner of a right-angled isosceles triangle 
at M^hose acute angles are the flames. On looking into the plate, one flame and the 
image of the other are seen side by side, and ought to be of equal brightness. 
The film, however, by gradual tarnishing gi’adually becomes more transparent, so it 
is best to slightly overdo the plate, and let it age till right. Or an overdone film may 
be thinned down with potassic cyanide if wanted quickly. The unpermanence of these 
plates is a little troublesome; I should prefer to deposit a thin platinum film by 
“ electrical evaporation,” after the manner of Mr. Crookes. 
Certainly the bands can be seen wdien the images are very unequal, but they are on 
a background of spurious or non-interfering illumination, and for measurement it is 
desirable to get the bands exceedingly sharp. 
The unsilvered side of the semi-transparent plate, of course, reflects some light, and 
gives another image. With a short course for the beam this useless image comes 
into the field, but it need not be superposed on the other unless the plate happens to 
be of very uniform thickness ; and, if superposed, it can be got rid of by using a Nicol. 
With a very long course for the beam a Nicol is useless, for all the light seems 
polarized after so many reflexions ; but fortunately, although the plate should be of 
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