ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 641 



with condensers arranged for object slides, having a thickness of 2 mm. 

 Fig. 105 shows a Liliput lamp with hood, as adapted for continuous 

 current, and giving a light of about 500 candle power. 



Divergent Microscopical Amplifier.* — A. Berget points out the 

 inconvenience sometimes felt, owing to the excessive shortness of frontal 

 distance, in the study of an object" under a high power. Having had 

 occasion to experience this difficulty, it occurred to him that the difficulty 

 might be overcome by the interposition of a divergent system. His 

 method is shown in fig. 106, one (or more) divergent lenses of short 

 focal length being inserted in a tube sliding with body of the Microscope. 

 The system must be inserted in such a manner that the real image a/3, 

 which the objective would give were it not for the divergent lens, may 

 be placed between this lens L and its focus F. Under these conditions 

 it acts as a " virtual object " and gives an image real, enlarged and of the 

 same sense, A'B', of the object AB. It is this image A'B' which is 

 observed by the help of the ordinary ocular of the Microscope. In 



Fig. 106. 



illustration of his method the author found that with a No. 3 objective 

 focused on a micrometer divided into hundredths of a mm., the 

 ocular being provided with a thread governed by a micrometer screw and 

 divided drum, it was necessary to rotate the drum-head eight divisions 

 in order to measure an interval of 10 /x,. But when the divergent 

 system was interposed, everything else remaining unchanged, a dis- 

 placement of 60 divisions on the drum-head was necessary. In other 

 experiments the frontal distance was about 1 cm. To have obtained the 

 same magnification without the divergent system, a No. 7 objective would 

 have been necessary, with a frontal distance equal to a small fraction of a 

 mm. The author considers that his method could be usefully applied in 

 many cases. 



Apparatus for Measurements of the Defining Power of Objectives.! 

 The theory of the distribution of intensity in the image formed by a lens 

 with circular aperture has been discussed by Lord Rayleigh in his article 

 on " The Wave Theory of Light," in the Encyclopaedia Britannica,} 



* Comptes Rendus, cxlviii. (1909) pp. 1097-9 (1 fig.). 



t Proc. Roy. Soc, Series A, lxxxii. (1909) pp. 307-14 (5 figs.). 



j See also Collected Papers, iii. 



