538 REPORT— 1902. 



• 



2. That rays should pass through the calcite lens parallel to its axis, Avliich 

 should correspond with the axis of the crystal. 



3. That the lens should be achromatic for the wave-lengths chosen. 



4. That as far as possible the combination should be aplanatic, but in any case 

 the aberration should be the same for both wave-lengths. 



5. That only two lens tools should be necessary, so that it may not be 

 expensive. 



These requirements are satisfied by a triple consisting of two outer bi-convex 

 lenses of quartz enclosing an equi-conciave ot calcite. The details follow :— 



Radii. R and S" = ± 2-10739. S, R', S' and R" = ± 1-0541G. 

 Thickness, t and t" - -33703 (quartz), t' - -0750 (calcite). 

 Diameter = 1 .50. 

 Mean principal focus = 2-338!)7 



„ conjugate ., = 4-67748 for wave-length 2748 



= 4-67839 „ „ 5607 



Outstanding chromatic error = -00091 



Spherical aberration wave-length 2748 = — -78027 



5607 = - -81762 



The spherical aberration may be reduced to vanishing-point for the heights 

 chosen by separating the first two lenses, but owing to the loss of light by reflec- 

 tion at the air-space it is preferred to make optical contact by cementing all inner 

 surfaces with glycerine. The spherical aberration is not as serious as would 

 appear, the height taken near the vertex representing a very small area. 



To avoid double refraction the distances of spark and object from the lens 

 must be the same. 



It is best to have one right-lianded and one left-handed quartz element. 



t'alcite becomes opaque to ultra-violet beyond wave-length 2145, but the skin 

 and tissues are already very opaque at wave-length 25(J0. 



7. J/r. Fetavel's Recording Gauge ajyplied to Ordnance and Small Arms.^ 

 By Captain J. Bruce-Kingsmill, R.A. 



After a brief consideration of some of the problems of ballistics, an account 

 was given of the various methods by which in modern practice the properties of 

 explosives are determined. 



Although the oldest type of heat engine, the gun was till recently the least 

 well understood. It was not until the crusher gauge had been brought out by 

 Sir Andrew Noble (then Captain Noble, R.A.), when a member of the Explosives 

 Committee of 1860, that ttie maximum pressure could be accurately measured. 



Thanks to his exhaustive researches and those of Abel, Vieille, and Berthelot, 

 thanks also to the uninterrupted experimental work which has been carried out 

 independently at such centres as Woolwich, Elswick, Essen, by the Creusot 

 Company, &c., the necessary data have gradually been determined, and the design 

 of artillery (which at one time was a matter of guesswork and rule-of-thumb) 

 has become an exact science. 



But the use of higher pressures and more powerful artillery has of late made 

 a further advance necessary. 



The rapid development of the steam-engine and the high efficiency attained to 

 by it may to a large extent be attributed to the use of the indicator by means of 

 which the internal working of the engine can be ascertained. 



The need of a corresponding device for use with artillery was soon felt. 



To meet this want several instruments have been devised. 



Sir Andrew Noble conceived the ingenious idea of relieving the spring of the 



' Published in extensu in Engineering, March 13, 1903. 



