276 SUMMARY OF CURRENT RESEARCHES RELATING TO 



horizontal movement of tlie spectroscope before described, then in tlie 

 first case there is a spectral region found towards the red end in 

 which the doubly refracting object appears dark upon a bright ground ; 

 but in the second case such a region appears at the violet end, because 

 with augmented or diminished thickness of the plate, the value of A 

 in the equation (2) is altered. With increasing thickness of the 

 doubly refracting plate, the dark interference-bands move in the 

 spectrum from the violet to the red end, and vice versa with diminish- 

 ing thickness. 



By this method small degrees of double refraction in organized 

 bodies can be more certainly discovered, and for certain histological 

 objects a very safe opinion as to their double refraction can be arrived 

 at. Dr. Eollet has employed the method especially for the examina- 

 tion of striated muscle-fibre, and obtained very good results as to 

 the double refraction of the transverse, the accessory, and the 

 terminal or intermediate disks. If we place a striated muscle-fibre 

 upon the slide so that its longitudinal axis coincides with es — es 

 (fig. 45) it will be in the so-called " addition-position " above the 

 selenite film ; all its above-mentioned doubly refracting parts will 

 therefore be brightly illuminated in the dark interference-band. If, 

 however, the spectrum is so moved that the spectral regions near the 

 red end lie under the fibre, we obtain an image in a given region 

 which is, as it were, the negative of the former, because all parts of 

 the fibre which before appeared bright on the dark ground of the 

 interference-band, now appear dark upon a bright ground. The 

 second image thus checks the first. 



Moreover degrees of double refraction may, in some cases, be dis- 

 tinguished by the extent of the movement of the spectrum which is 

 necessary to obtain the negative image. If the selenite film is turned, 

 while the fibre remains in the direction of the slit, so that not e e as 

 in fig. 45, but o o, falls in the direction s s, then the fibre will lie in 

 the " subtraction-position " above the film. The position in the 

 spectrum of the interference-band of the film remains unchanged and 

 the doubly refracting parts of the fibre shine as before on the dark 

 ground. But now, in order to obtain the negative image, the spectrum 

 must be so moved that a spectral region nearer the violet end lies 

 under the muscle fibre. By this apparatus, therefore, addition and 

 subtraction positions can be directly distinguished from one another. 



With regard to the selenite films, the author remarks that these 

 were chosen because they are easily replaced and are abundant in 

 commerce. In the selection of the particular films mentioned above 

 the position of their interference-bands was determined in the central 

 part of the spectrum, so that there was the necessary space between 

 that and the red and violet ends. It is, however, clear that exactly 

 similar observations can be made with interference-bands in other 

 spectral regions and of other orders, by the employment of thicker 

 or thinner films than those which correspond to the red of the first or 

 second order. 



The use of interference-bands of a higher order is not suitable, 

 because the increase in thickness, which moves them to a ijroportionato 



