86 HAMILTON HARTRIDGE ON 



ment to be one-tentli (approximately) the diameter of tlie iris 

 of the illuminant. The iris can be varied from 20 mm. to 

 1 mm., and the field therefore altered from 2 mm. to 0"1 mm. 

 For high-power work smaller circles of illumination are some- 

 times required ; these may be obtained by dropping into the 

 iris grooved brass discs the centres of which have been drilled 

 with small apertures. In this way the illumination may be 

 limited to O'Ol mm. diameter. Smaller apertures than this 

 would not serve any useful purpose because 0"01 mm. is ap- 

 proximately equal to the diameter of the maximum aberration 

 disc of the condenser system. The illumination with this ap- 

 paratus is found to be suitable up to one thousand diameters 

 magnification approximately, but for objective testing or very 

 high-power work greater illumination may be required. This 

 may be obtained by using a small half-watt with a frosted bulb, 

 the opal glass being omitted. For greater intensities still the 

 bulb may be coated with a thick deposit of silver, and then a 

 small aperture of the required size scratched away and then 

 frosted, so that the light issues from this as if from a source of 

 the required shape and size. The intensity otained in this 

 way is very great indeed, since nearly the whole of the illumina- 

 tion of the lamp has to find its way out through this aperture. 

 (I have used the same method for obtaining a fine linear source 

 of great intensity suitable for a monochromatic illuminating 

 apparatus with complete success ; details of this apparatus will 

 be given in a future paper.) The life of such a lamp does not 

 seam to be very long, but doubtless improvement could be 

 effected in this direction. It is possible that painting the 

 outside of the silver coating with dead black varnish would 

 facilitate the radiation of heat, and thus prolong the life of the 

 filament. An alternative and apparently equally good method 

 would be to place an ordinary bulb within a metal box, the 

 inside surface of which has been silver plated and highly polished. 

 This box would be perforated with an aperture of the required 

 shape and size. The outside of the box could be ribbed and 

 painted dead black, so as to keep the box cool. 



For work (usually with low powers) where the use of an oil- 

 immersion condenser may be considered unnecessary, I employ 

 a dry uncorrected Abbe condenser, which is fitted with a pair of 

 auxiliary lenses, so as to employ the same source of light and 



