THE AID OF THE ACHROMATIC FRINGES. 



43 



suggests itself, so that a line line may be seen in the ocular, normal to the scale. 

 In the case of parallel rays, however, the displacement of the image in the 

 ocular would be no larger than the displacement of the objective, /, figure 41. 

 To obtain increased displacement, the method of figure 43 is available, where 

 S is the fine slit in front of a Welsbach burner at B. At d 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 5, the intercept D will represent the displacement 

 in the ocular. If the distance Sd be a and dD, b, we may write 



(i) 



1+1 = 1 

 a 6 



where / is the principal focal distance of the objective. Hence 



D b 



0) 



f 



3 



Theoretically, therefore, any degree of magnification is possible by increas- 

 ing b (the distance of T and/, fig. 42) and decreasing/. In the former case, 

 some means of controlling the thumb-nut m, figure 41, 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 I did later, 

 particularly since a lens-breadth of a few millimeters will suffice ; for there is 

 an abundance of light. 



To measure the width of the band of light produced by the vibration, the 

 ocular T may be on an axle t, with slight friction. The zero of the micrometer 

 scale may then be brought to coincide with one edge of the band by rotation 



