138 



THE AMEEICAN MONTHLY 



[Jniy, 



Nolan are secretaries of the local 

 committee, Philadelphia. 

 o 



The Electric Light in Micro- 

 scopy. — Last month we referred at 

 some length to the use of the electric 

 light for illuminating objects under 

 the microscope, and proposed this 

 month to treat briefly of another part 

 of the subject. Dr. Max Flesch * has 

 written an able article in which the 

 electric light is considered as to its 

 value as a source of light for this pur- 

 pose. 



The value of a light for microscop- 

 ical purposes can be determined by 

 considering the causes which influ- 

 ence the capabilities of a microscope. 

 It is well known that greater resolv- 

 ing power is obtained with blue than 

 with white light. Accordingly, it iS 

 the quality of the light — the relative 

 proportion of light-waves of short 

 wave-length, or, as we may say, of 

 rays near the blue end of the spec- 

 trum — that must be considered. 

 ' The limit of resolution of the mi- 

 croscope, which cannot be extended 

 under present conditions, depends 

 upon the illumination, and under the 

 most favorable conditions it does not 

 exceed with the most oblique light |, 

 or with perfectly central light f , of a 

 wave-length (about 0.55 ft) of white 

 light. With illumination by homo- 

 geneous blue light of about 0.43 fi 

 wave-length (Fraunhofer's line g), 

 under the same circumstances, the 

 limits above expressed become re- 

 duced to about ^\ and j\, respect- 

 ively ; that is, to about 0.15 /* and 

 30. ft. The possibility which the fore- 

 going indicates to increase the capa- 

 bilities of the microscope by the use 

 of blue instead of white light makes 

 it desirable to introduce illuminating 

 apparatus which permits of the 

 ready application of monochro- 

 matic light. Upon these grounds 

 Dr. Van Heurck has already applied 

 the electric light to microscopical 

 work with excellent results, and he 



* Zeitscr. fur Wt'ss. Mikr. 



has thus explained why it is better 

 for the study of minute details than 

 the yellow light of an ordinary lamp. 

 In the article by Dr. Flesch this sub- 

 ject is considered at some length with 

 reference to the proportion of blue 

 rays available in practice. As is 

 well known, the proportion of rays 

 of different colors emitted by an in- 

 candescent body 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 will vary with the 

 strength of the current. E. O. 

 Meyer, who has published an article 

 entitled ' On the Color of the Electric 

 Light,' gives the following propor- 

 tions for the different lights examined 

 by him : — 



Incandes- 



Arc Light, cent Light. Gas Light. 



Red 2.09 1.48 4.07 



Yellow 1. 00 1. 00 1. 00 



Green 0.99 0.62 0.43 



Blue 0.87 0.21 0.23 



Violet 1,03 0.17 0.15 



Extreme violet.. ..1. 21 — — 



The above table represents the 

 brightness of the several lights com- 

 pared with that of the sun, the latter 

 being reduced in intensity until the 

 yellow was the same in each case. 

 The incandescent lamp employed 

 was one of Edison's form. 



It will be seen that the incandes- 

 cent light contains, relatively, more 

 of the blue rays than gas light. This 

 light, it may be concluded, possesses 

 the advantages of ready application, 

 comparatively little heat, considerable 

 intensity, and an abundance of rays 

 of short wave-length. It may readily 

 afford monochromatic light, without 

 undue loss of illumination. 



Another great advantage of this 

 light, which would seem to commend 

 it, especially to those who use the 

 microscope in lithology, and also for 

 the examination of stained prepara- 

 tions, is the color of the light, which 

 shows colored objects nearly the 

 same as they are seen by daylight. 



Last month we suggested that deal- 

 ers might do well to provide electric 

 lamps suitable for use with the mi- 



