126 



SUMMARY OF CURRENT RESEARCHES RELATDTG TO 



aberrations, a comparison of the coloured borders of the separate 

 partial images, and an examination of their lateral separation and 

 their differences of level, as well in the middle as in the peripheral 

 zones of the entire field, suffices for an accurate definition of the 

 nature and amount of the several errors of correction, each of them 

 appearing in its own primary form. Therewith we also see that 

 which arises from aberration, proj^erly so called (faults of focussing 

 function), clearly separated from such imperfections or anomalies as 

 spring from mere differences of amplification between unequally con- 

 verged and unequally refracted rays ; and moreover we eliminate 

 completely all influence of the ocular on the quality of the image. 



Hardy's Chromatoscope. — Mr. J. D. Hardy describes a method of 

 illumination by an instrument (figs. 20 and 21) which he calls the 

 " Chromatoscoije " : — " Its chief purpose is that of illuminating and 

 defining objects which are non-polarizable, in a similar manner to that 

 in which the polariscoj)e defines polarizable objects. It can also be 



Fig. 20. 



Fig. 21. 



CH 



d^ 



Sp. Spot lens in its tube St. Ch. Chroma- 

 toscope glass plate resting on the 

 inner flange of the tnbe A. 



The letters indicate the disposition 

 of the blue, red, and green stained 

 glass. 



applied to many polarizable objects. This quality, combined with 

 the transmission of a greater amount of light than is obtainable by 

 the polariscope, renders objects thus seen much more efifective. 



It is constructed as follows : — Into the tube of the spot lens 

 (fig. 20) a short tube is made to move freely and easily. This inner 

 tube has a double flange, the outer one (which is milled) for rotating, 

 and the inner one for carrying a glass plate. This plate (fig. 21) is 

 made of flat, clear glass, and upon it are cemented by a very small 

 quantity of balsam, three pieces of coloured (stained) glass, blue, red, 

 and green, in the proportion of about 8, 5, and 3, as shown in the 

 figure. The light from the lamp is allowed to pass to some extent 

 through the interspaces, and is by comparison a strong yellow, thus 

 giving four principal colours. Secondary colours are formed by a 

 combination of the rays in passing through the spot lens. The 

 stained glass should be as rich in colour and as good in quality as 

 possible, and a better effect is obtained by three pieces of stained 

 glass than by a number of small i^ieces. 



The application of the chromatoscope is almost unlimited, as it 

 can be used with all objectives up to the l-8th. Transparent objects, 



