766 Prof. H. Rubens and Mr. H. Hollnagel on 



» 



simply split from a large cube of transparent salt and care- 

 fully ground and polished to an optical surface. 



The necessary plates for the Reststrahlen of sylvine 

 were obtained through the kindness of Prof. Dr. Heinrich 

 Precht, Director of the Neu Stassfurter Salzbergwerk, who 

 kindly furnished the valuable material. We take this 

 opportunity of expressing our sincere gratitude for this 

 important aid in furthering the work. The block from 

 which the four plates, of dimensions 2 x 10 x 10 cm., were 

 cut was of a crystalline structure and milk-white turbidity. 

 It consisted of 98'5 per cent, potassium chloride and 1*5 per 

 cent, sodium chloride. It proved to be capable of re- 

 ceiving a very fine polish, and also retaining it for several 

 weeks. 



To produce the Reststrahlen of potassium bromide and 

 potassium iodide cast plates of these salts were utilized. 

 These were obtained as follows : — A quantity of salt was 

 melted in a nickel crucible in a large gas furnace ; when of 

 sufficient fluidity the molten mass was poured into a brass 

 vessel of the requisite dimensions — 10 x 10 x 1*5 cm. — and 

 allowed to cool. On crystallization, such blocks were very 

 porous, uneven, and overrun with cracks : however, they were 

 rendered plane very nicely by turning in the lathe. No 

 difficulty was experienced in grinding and polishing the 

 potassium bromide surfaces; on the other hand, the potassium 

 iodide plates were not rendered useful till after a series of 

 most fruitless attempts. The latter as they came from the 

 lathe showed a most porous structure, which increased with 

 continued grinding. Good surfaces were ultimately ob- 

 tained by grinding on each other, until dry, two surfaces 

 originally moistened with water. Under these conditions a 

 saturated solution of the salts formed between the surfaces in 

 contact ; this, entering the orifices and cracks, crystallized 

 out immediately and filled them with solid salt. Such sur- 

 faces when carefully ground could be polished sufficiently 

 well for the present purpose with the palm of the hand, 

 diamantine powder, and paraffin oil. Mirrors thus prepared 

 maintained their polish several da} r s. It seemed of greater 

 importance to eradicate the porosity and unevenness of the 

 surface, rather than to strive to obtain an optically perfect 

 mirror, for, according to earlier researches *, the reflectivity 

 of a surface for waves of greater length, is more and more 

 independent of the polish as the wave-length increases. To 

 this end the above method is sufficient. That surfaces of 



* H. Euhens and E. F. Nichols, I. c. p. 447. 



