.572 Notes. 



Possible Directions of Progress in Optical Glass. 

 By Walter Bosenhain, B.A. B.C.E. 



The author thinks there ought to be no slackening in the demand for 

 further advance in optical glass. The progress due to Schott and Abbe did 

 not in all probability exhaust the possibility of further useful optical 

 material, and a wide extension of the available range would be likely to lead 

 to great advances in optical systems. 



The optical constants, such as the refractive index, of the glasses at the 

 opticians' disposal at present were confined within distinctly narrow limits, 

 and there was probably some physical fact to account for this. 



Eeasons are given for supposing that all fluids may be made to solidify in 

 a vitreous amorphous state or in a crystalline state, according to the way in 

 which the solidification is allowed to take place — theoretically it was a 

 question of the rate of cooling and of the absolute pressure. With glasses 

 having extreme optical properties, it was found that the tendency to 

 crystallisation increased, and the methods are discussed by which this 

 tendency may up to a certain point be overcome. But a definite limit is 

 soon reached owing to the impossibility of fulfilling in practice the theoretical 

 conditions. Another limit was imposed by the fact that glasses of extreme 

 optical properties were also of the nature of active chemical agents, both in 

 the fused and in the ordinary solid condition — in fact, a large number of 

 glasses having most desirable properties had to be eliminated from this 

 cause. Owing to these considerations, the author draws the conclusion that 

 any considerable extension of the range of available optical glasses is not 

 likely to be made on lines at all analogous to those pursued in the production 

 of glasses, but that the most promising direction of progress was to be found 

 by accepting the limitations discussed, and in fact taking the line of advance 

 indicated by the most serious of those limitations, viz., the tendency to 

 crystallisation. The object, then, to be aimed at was the production of 

 crystals of composition and properties suitable for optical uses. The task, 

 though exceedingly difficult, should not be more so than was the problem 

 of producing homogeneous optical glass in large masses to the men who 

 attacked that problem a century ago. The optical behaviour of the same 

 substance in the vitreous and crystalline condition always differed greatly. 

 The fact was known in the case of silica, and had been found in a marked 

 degree in experimental glasses produced by the author, having a chemical 

 composition identical with that of certain minerals. For that reason the 

 author emphasises that he does not advocate attempting to get suitable 

 crystals by the "devitrification" of extreme optical glasses, nor of novel 

 glasses by imitating the composition of minerals with promising optical 

 properties. What was necessary was to proceed by studying the conditions 

 to be fulfilled by a crystalline material for optical purposes. The need for 

 transparency and exclusion of all colouring oxides ruled out the great 

 majority of natural minerals. Transparency likewise necessitated that the 

 crystals should be of sufficient size, as crystalline aggregates were useless. 

 Double refraction was objectionable, therefore they were restricted to 

 materials which crystallise in the regular system. To investigate the optical 

 properties of these, a beginning might be made by a detailed study of the 

 optical properties of natural minerals. From a table given, which shows 

 some of the optical properties and chemical composition of natural minerals, 

 taken from Bosenbusch's " Hiilfstabellen zur mikroskopischen Mineral- 

 bestimmung in Gesteinen," it would be seen that a very considerable 

 extension of optical properties would be made available by artificial pro- 

 duction of similar materials in a suitable form. Suggestions are then made, 

 how intermediate forms, extreme forms, and colourless analogues of the 



