SI IS 



SUMMARY OF < TKKKNT RESEARCHES I {ELATING TO 



13) Illuminating- and other Apparatus. 



New Nernst Lamp for Microprojection.* — A. Kdhler, in discussing 

 some of the difficulties attendant upon the Nernst lamp, points out thai 

 ilic besl results are attained when the collector-lens is so shaped that its 

 image of a luminous bar completely covers the actual aperture of the 

 iris-diaphragm : in other words, if the breadth of the image of aluminous 

 bar is at least equal to the diameter of the diaphragm aperture Thus. 

 if (fig. 89) 2L 1 be the breadth of the image of a bar, or, generally, the 

 least diameter of the image of the light-source, 2?* the diameter of_ the 

 condenser-diaphragm, then this condition will be satisfied when L 1 ^/ - . 



If, further, 2L be the breadth of a bar, or, generally, the least 

 diameter of the light-source, f\ the focal distance of the collector-lens. 



Fig. 89. 



and x l the distance of the condenser stop from the rear focal point F 1 of 

 the collector, then 



L 1 = .r l 



From the foregoing results it follows that, by taking only the equality 



sign in the first equation, 



r ; .i 



L ' A 



and this derived equation gives approximately the distance at which a 

 collector of focal distance f x belonging to a Microscope of assigned optical 

 structure should be set up. If the image of the light-source is also to be 

 sharp, when the collector has a greater angular aperture, there must be 

 aplanatism ; this requires that, 



L sin u x = L 1 sin u { 



The author proceeds similarly to investigate other conditions, with 

 especial reference to Zeiss' apochromats and compensation oculars. His 

 principles have been carried into practice by Messrs. Carl Zeiss, and a 

 view of their apparatus is shown in fig. 90. It has the advantage, in the 

 case of small light-sources, of combining the collector-lens with the 

 Nernst. The collector, provided with an iris-diaphragm 1, is clamped 



* Zeitschr. wiss. Mikrosk., xxvii. (1911) pp. 477-88. 



