212 
PHYSICS: C. BARUS 
The lens carriage gg' is then attached with cement (after the wires are evenly- 
stretched) with the crosswires r, s on the opposite side of dd' to the pull of the 
magnet i. The magnet, in addition to the cement, thus guards against slip- 
ping. On turning the screws b and b' any degree of tension may be imparted 
to the wires bb' , roughly. This simple device worked surprisingly well, and 
wires of different kinds may be easily inserted or replaced, the lens system be- 
ing subsequently attached with cement; but it is better to loop the lower part 
of dd' around a special roller G, as indicated in figure 2, and used in my later 
tests, with the object of more easily reaching an equality of tension in the 
wires d and d\ The tensions are then roughly changed by the screw and 
nut u. 
The approximate tension having thus been obtained by the screws u, 5, 
b\ the finer variations are imparted by the screw mn which flexes the elastic 
rectangle ABCDE and thus gives to the bifilars d, d' exactly the tension 
required. It is at the thumb nut m that all adjustment is made during 
observation. 
In my apparatus the rectangle was about 50 cm. long and 12 cm. wide. 
The wires d, d' about 1.5 cm. apart and each about 0.025 mm. in diameter. 
Wires as thick as this require sharp adjustment as to tension, but the method 
given proved quite satisfactory particularly as it is little disturbed by manipu- 
lation. The tension is sufficient to admit of an air gap of less than a milli- 
meter between the plate g^ and the magnet i of the telephone. Later the tele- 
phone was also put on a spring-micrometer screw for fine adjustment. 
In the case of parallel rays, the displacement of the image in the ocular would 
be no larger than the displacement of the objective, /, figure 1. To obtain 
increased displacement, the method of figure 3 is available where 5 is the fine 
slit in front of a Welsbach burner at 5. At is the principal plane of the 
vibrating objective and at D the micrometer plate in the ocular. Again, if the 
length d represents the double amplitude of the objective and the sides of the 
triangle be drawn from 6", the intercept D will represent the displacement in 
the ocular. If the distance Sd be a and dD^ b, we may write 
, l/a + 1/6 = 1// (1) 
where / is the principal focal distance of the objective. Hence 
D/d = b/f (2) 
Theoretically, therefore, any degree of magnification is possible by increasing 
b (the distance of T and /, fig. 2) and decreasing /. In the former case, some 
means of controlling the thumb nut m, figure 1, from a distance would have to 
be provided. In the latter the lens / would have to be achromatic. In the 
present experiments I first used an ordinary spectacle lens at / with a slotted 
screen between it and the slit to diminish chromatic aberration; but there is no 
objection to the use of an achromatic lens at/, as was done later, particularly 
since a breadth of a few millimeters of lens will suffice, for there is an abun- 
dance of light. 
