374 Messrs. du Bois and Rubens on Refraction 



In many cases, where the object is rather qualitative, it 

 further appears advisable to work at considerable inclination; 

 the fittest angle is 60° ; the deviation is then increased about 

 three-fold. The apparent breadth of the prism, however, is 

 only halved (cos 60°= 0*500), so that the diffraction effect 

 does not yet present too great an obstacle. As remarked 

 above, however, refractive indices can only be calculated from 

 measurements with normal transmission ; we have therefore ad- 

 hered to the latter method for our measurements of dispersion. 



§ 15. Other Metals. — These remarks can only apply to 

 metals having a high index, such as those now examined. 

 For these metals one condition is always fulfilled, which 

 appears important to us, viz. that the values of i m hardly 

 exceed 30°, an angle the cosine of which is still near unity*. 

 For Fe and Co i m even remains below 20° ; accordingly the 

 deviations from Snellius's law are most considerable for Ni, 

 which has the smallest index. 



We have also given theoretical curves for silver and gold in 

 fig. 3 ; we began by assuming the sine law to hold and took 

 the indices given by Kundt (A, p. 266) : Ag (white) 0*27 ; 

 Au (white) 0*58 ; Au (blue) 1*00. Of course in the latter 

 case a deviation should never occur, and i m — i always. 



These theoretical curves show that for i >15°"66 (Ag) and 

 i > 35°*45 (Au^, total reflexion ought to commence ; greater 

 angles of emission into air would then be impossible. The 

 deviation curves in fig. 2 have been calculated for prisms 

 with a refracting angle /3 = 25"; they should more properly 

 lie under the axis of abscissa, as the deviations all become 

 negative. The curves show that for these metals also the 

 numerical values of a ought to increase with i, finally to reach 

 the value /3 for the critical values of the abscissa given above 

 (see equation (4) § 10) • at this point the curves must come 

 to a dead stop. 



§ 16. It is evident, however, from the most casual observa- 

 tion that silver or gold films may be looked through at any 

 angle. This shows that the assumption of Snellius's law, 

 made above for the sake of further discussion, must be quite 

 beside the truth in this case, even as a rough approximation. 



For different reasons it appeared to us unadvisable at 

 present to attempt to pursue this question experimentally. 

 It may be indeed foreseen that the advantages offered by 



* 4-S is seen from fig. 3, im hardly increases any more when i increases 

 from 65° to 90° : we should therefore probably have learned nothing more 

 even if we had been able to push the inclination beyond (55°. 



