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an 3niproveb fIDcanB of obtainino Critical 

 3lluniination for tbe flDicroecope. 



By Henry G. Piffard, M.D. 



CRITICAL illumination is that sort or kind of illumination 

 which best conduces to the revelation of the intimate 

 structure of microscopic objects. The illumination is said 

 to be " critical " when the image of the radiant (lamp-flame or 

 otlier source of illumination) is brought to a focus by mirror or 

 condenser at the plane of the object under examination. Skilled 

 raicroscopists are pretty well agreed that the most convenient and 

 feasible means of obtaining critical illumination is by focussing 

 the edge (not the flat side) of a half-inch kerosene flame on the 

 object. 



The writer is unable to use with comfort either gas or an oil 

 lamp in microscopic work, but has found that he can work by 

 electric light for several hours continuously without inconvenience. 

 Attempts to obtain satisfactory critical illumination from this 

 source have occupied a portion of his time during the past two 

 years. Without referring to the devices he has abandoned, he 

 will simply describe the one at which his experiments have ter- 

 minated, leaving to others the opportunity to still further 

 improve it. 



The ordinary electric lamp in domestic use has an illumin- 

 ating value of sixteen candles, and its thread-like filament is 

 brought to incandescence by a current having an intensity of 

 about half an ampere under a pressure (in the Edison system) of 

 from one hundred and fifteen to one hundred and twenty volts. 

 The light is distributed over a filament about five inches in length. 

 Such a light is not a desirable one for microscopic work. It 

 would be much better to have the light more condensed by the 

 use of a shorter and thicker filament. There is no difficulty 

 whatever in constructing such a lamp, but if it were brought 

 directly into the Edison circuit, its life would be exceedingly 

 brief ; in other words, a certain length of filament is required to 

 withstand the pressure of the current from the main. Two things, 

 then, are needed : First, a lamp with a short and thick filament ; 



