968 SUMMARY OF CURRENT RESEARCHES RELATING TO 



preparations by its peaceful light, undisturbed by troublesome 

 shadows of the hand and instrument. 



As the spectrum of the incandescent lamp is not only continuous 

 but incomparably more intense in the blue and violet than that of any 

 other artificial light, the suitability of the light was tested for spectral 

 absorption analysis. Complete success was obtained in recognizing 

 in the blue and violet the absorption-bands of such colouring matters 

 as had hitherto only been capable of investigation with sunlight ; for 

 instance, the three between F and H of the yellow colouring 

 matter of the yolks of eggs, the alcoholic-ether extract being placed 

 between the slit of the spectroscope and an Edison lamp. 



Dr. M. Flesch also considers * the advantages of the electric light 

 for microscopy. 



The value of a light for microscopical purposes can be judged of by 

 considering the causes which determine the maximum capacity of the 

 Microscope. " The limit of resolution of the Microscope, which under 

 present conditions cannot be extended, depends upon the illumination, 

 and under the most favourable conditions it does not exceed with the 

 most oblique light 3/8, or with perfectly central light 3/4, of a 

 wave-length (about • 55 /u,) of white light. With homogeneous blue 

 light of about • 43 /a wave-length (Fraunhofer's line G), under the 

 same circumstances, the above limits become reduced to about 3/10 

 and 6/10 respectively ; that is, to about 0*15 ju, and 30 (x." f The 

 possibility of thus increasing the resolving power of the Microscope 

 by the use of blue instead of white light, makes it desirable to 

 introduce illuminating apparatus which will permit of the ready 

 application of monochromatic light. It follows from the preceding 

 that a good microscopic lamp must be rich in blue rays. This in the case 

 of incandescent bodies is dependent upon the temperature. At 1500° C. 

 bright blue rays are emitted, at 2000° violet rays. In the case of the 

 electric light, the proportion of short-wave rays will vary with the 

 strength of the current. 0. E. Meyer ;{: gives the following table 

 showing the brightness of the different lights, compared with that of 

 the sun, the latter being reduced in intensity, through polarization, 

 until the brightness of the yellow light was the same in each case. 



Arc Light. ^^^^fSn'^!"'*- GJaslight. 



Eed .. .. 2-09 .. 1-48 .. .. 4-07 



Yellow .. .. 1-00 .. 1-00 .. .. 1-00 



Green .. .. 0-99 .. •0-62 .. .. 0-43 



Blue-green ,. — .. 0*29 .. .. — 



Blue .. .. 0-87 .. 0-21 .. .. 0-23 



Violet .. .. 1-03 .. 0-17 .. .. 0-15 



Extreme violet .. 1*21 .. — .. ,. — 



The incandescent light contains, it will be seen, relatively, more of 

 the blue rays than gaslight ; and it will, therefore, much facilitate 



* Zeitschr. f. Wiss. Mikr., i. (1884) pp. 175-81. 

 t Dippel's ' Das Mikroskop,' i. (1882) p. 324. 

 X Oentralbl. f, Elektrotechnik, v. p. 457. 



