364 SUMMARY OF CURRENT RESEARCHES RELATING TO 



3. Lastly, as the power is also the enlargement produced at unit 

 distance from the posterior focus of the objective, it can be evaluated 

 directly by determining the position of this posterior focus (i.e. the foco- 

 facial posterior distance), then by measuring the enlargement produced 

 at a decimetre further, or at any other unit of distance. 



The author gives a sample table of results obtained with a certain 

 number of objectives of different strengths. 



Cheap Glass Lenses.* — A cheap and simple way of obtaining large 

 lenses suitable for photographic and such-like work, is to form a 

 combination of ordinary watch, or clock glasses, with water or other 

 •suitable liquid. Two glasses, whose edges must be well ground so as to 

 fit well when placed in contact, are dipped into the liquid and removed, 

 filled with it, as a whole. The edges are then wiped dry, and moistened 

 with water-glass, which, helped with a little hydrochloric acid, sets quite 

 hard, so that the " lens " can be freely handled. This process is assisted 

 by the use of a peculiar brush, having two pencils on one shaft. As 

 this brush is passed round the periphery, the front pencil wipes off the 

 superfluous fluid, and the following pencil applies the water-glass. By 

 means of a glass disk and a watch-glass, a plano-convex lens can be 

 made, and several other forms are possible. 



Bebger, E. — Ueber das bei meiner binokularen Lupe verwendete Linsen-system. 



Deutsche Mech. Zeit., 1905, p. 155. 

 Brass, A. — Die Linsenfassungen. 



[Discusses several errors sometimes made in lens-mounts, and 

 emphasises the necessity of lamp-blacking their inner sur- 

 faces.] Central. Zeit. Opt. Mech., xxvii. 



(1906) pp. 15-17 (10 figs.) 



,, ,, Die Zusammensetzung von Linsen system en. 



[A popular explanation of certain principles.] 



Tom. cit., Nos. 4-7, pp. 31-3 (2 figs.). 



(3) Illuminating: and other Apparatus. 



A New Application of the Abbe Condenser.f — F. K. Studnicka 

 reminds his readers that the Abbe condenser presents a real, reversed 

 image of the source of illumination. If an object, such as a micrometer 

 scale, be suitably interposed, an image of this will be similarly formed 

 on the stage, and may be applied to measuring an object there. In 

 this way, by the help of a proportionately stronger objective, a series of 

 magnifications can be obtained which are very convenient for drawing, 

 and for the preparation of various objects. The size of the image 

 formed above the front-lens (viewed from above) of the condenser 

 depends on the distance of the object from the under-lens ; the image 

 is smaller as the distance is increased. Thus it will be seen that the 

 series of magnified images thrown on to the stage will vary from zero 

 to a maximum. This image can be combined with various powers of 

 objective, eye-piece and tube-length, and thus the series of attainable 

 effects is practically infinite — although only observations in the middle 

 of the field will be free from sensible distortion. Instead of the con- 

 denser, weak achromatic objectives might be used. A difficulty would, 



* Central. Zeit. f. Opt. u. Mech., xxvi. (1905) pp. 261-2. 

 t SB. k. Bohm. Gesell. der Wiss., 1905 (iv.) 4 pp. 



