ILIA'MIXATinX 78 



has been calculated, and w hicli therefore has a large amount 

 of optical error. 



On account of the additional liability to errors in the 

 distancing and centering of an additional condensing lens, 

 the writer would not as yet recommend a corrected bull's 

 eye, separate from the microscope, for general use; and 

 does not use one himself. Direct flame and matt glass 

 seem to suffice for all work. An accessory achromatic 

 lens, however, must be fitted to all condensers corrected 

 for infinite lamp distance. 



Illumination of Uncorrected Condenser. — With the 

 uncorrected condenser, the diaphragms on the source 

 require to be larger than those for the corrected condenser. 

 That is, the image of the source on the object must be 

 larger than the object-field, for the high powers. The 

 resulting glare can only be lessened by cutting down the 

 condenser aperture by its iris. The simplest methods of 

 illumination of this condenser probably are: (1) an electric 

 bulb (C-Mazda), matt inside, with diaphragms close to it; 



(2) a strongly illuminated disc of matt glass in the dia- 

 phragm carrier of the condenser (Zeiss, Hartridge) ; and 



(3) a round flask filled with blued water, for a small source 

 (Zeiss). Of these three methods, the second is regarded 

 as the best. The focus in the first method is to be arranged 

 so as to get the largest continuous fight circle on the back 

 of the high-power objective {see Watsori's Microscope 

 Record, No. 16). 



Literature. — Instructive accounts of microscopical illu- 

 mination are to be found in the writings of Nelson (101), 

 Coles (46), Hartridge (71), and Beck (32, 33). Gage (66) 

 gives a good account of electric lamps. 



Summary. — An image of the incandescent source of 

 light thrown on the object gives unexcelled illumination. 

 This is probably done best, at present, with the oil-lamp 

 flame (Nelson, Coles), or the ribbon tungsten, for high 

 powers. In the absence of this, a finely ground glass 

 plate treated with magnesium sulphate, etc., with or 

 without a condensing lens between it and the lamp, suffices 

 for good work. It allows thin metal diai)hragras to be 



