348 BELL SYSTEM TECHNICAL JOURNAL 



be polished, only moulded, since here there is no focusing of light. The 

 filters described above are the Corning glass filters: 



7^1, Code 3484, H.R. Traffic Shade Yellow, 



F2, Code 5120, Didymium, 



Fz, Code 4303, Dark Shade Blue Green, 



HF, Code 3966, Extra Light Shade Aklo (a heat filter). 

 A corresponding set of filters passing more light, but giving a less mono- 

 chromatic light are: 



7^1, Code 3486, H.R. Yellow Shade Yellow, 



Fo, Code 5920, H.R. Illusion Pink, 



Fi, Code 4308, Light Shade Blue Green. 



It might be added that the pronounced effects of filters are easily observed 

 with any instrument by holding small polished filters over the eye. These 

 same filters give some improvement even with white light, for optic axis 

 detection. Further, if the filter were not so heavy, polished filters might be 

 better applied at the eye than at the hght source, since here they would also 

 cut down extraneous illumination from the room. Or the same result might 

 be obtained with large polished filters (expensive) and an eye chute at the 

 viewing end of the system. 



Several factors are of importance with regard to the immersion fluid used 

 in the inspectoscope. The fluid should have a refractive index matching 

 that of quartz, and be clear and colorless (to eliminate loss of light). It 

 should be of low viscosity, so that dirt and dust may settle and air bubbles 

 rise, rapidly (to prevent depolarization of the polarized light beam). Low 

 viscosity also aids in the draining of oil from the stones after inspection. 

 Water solubility of the fluid would be an aid to cleaning. Necessarily the 

 fluid must be non-toxic and non-flammable, and preferably odorless, inex- 

 pensive and commercial. Various fluids satisfying these requirements to 

 varying degrees have been used. Since there is no majority agreement as 

 to which of the fluids now in commercial use is most satisfactory, no particu- 

 lar fluid can be recommended. (Three are listed in Chapter II, page 258.) 



However, a word may be added about the required degree of refractive 

 index match. Mineral oils of index L47 to L48 are, definitely, very poor im- 

 mersion fluids for quartz. With them it is difficult to see into the interior 

 of stones without plane polished surfaces. Ground and unpolished surfaces 

 still cause considerable diffusion. For good inspection viewing the fluid 

 should have an index between 1.53 and 1.56 (preferably between 1.54 and 

 L55). 



The refractoscope is a simple instrument especially designed for the pur- 

 pose of easily and exactly checking the index match of fluid to quartz. • The 

 principle having been already noted (p. 255, Chapter II), it suffices here to 

 describe the use of the instrument. A test tube. Fig. 4.4, filled with the 



