176 SCIENCE PROGRESS 



light (Mikro 235), and (3) apparatus for the projection of spectrum experiments 

 (Mikro 205). 



(1) It is customary when illuminating an object for microscopic vision to use 

 a condenser below the stage of the microscope. This is really a microscope 

 objective of focal length 1-2 cm. provided with a large numerical aperture. When 

 daylight is used a reduced image of a cloud or patch of light in the sky is pro- 

 jected on to the object, the image produced being larger than the area of the 

 object. In this case uniform illumination is readily obtained. 



The use of an artificial source of light, however, is attended by certain 

 difficulties. The condenser projects an image of the source on to the object, but 

 this image is usually too small, and the intensity of the illumination is too unequal 

 to result in uniform illumination of the field of view. 



These difficulties have been overcome by the use of a collective-lens system 

 of great light-gathering power. It consists of three lenses, but for the highest 

 magnifications one lens is sufficient, being used as a simple collector. This lens is 

 adjusted so as to project a magnified image of the source on to the iris-diaphragm 

 of the condenser of the microscope. The full aperture of the collector acts as a 

 uniformly illuminated surface obviating the difficulty arising from irregularity of 

 illumination. For high-power dry and immersion systems the field of view of 

 the microscope is very small ; this single lens is sufficient for producing illumina- 

 tion of the whole object. For lower-power work, in which the field of view is 

 larger, the other two lenses of the system are also used. " The diameters of the 

 lenses are chosen of a sufficiently large size, so that in most cases the images 

 of the aperture of the collector, projected by the microscope condenser, illuminate 

 a larger area than can be included by the microscope objective in the magnification 

 concerned." This collecting system is of great use in projection experiments and 

 in photo-micrography. 



(2) Perhaps the most important feature of the apparatus for the projection 

 of experiments on polarised light is the small size of the Nicol prisms ; this is 

 made possible by an ingenious arrangement of lenses. In the projection of the 

 phenomena, which doubly refracting specimens show between crossed or parallel 

 Nicols, certain difficulties arise unless large prisms are available : this is due to 

 the necessity of illuminating the specimen in its entirety. The illumination can 

 be conveniently produced by placing in front of the specimen to be examined a 

 collecting lens at the focus of which is a small but powerful source of light. The 

 polarising Nicol is then inserted between the lens and the object. It can easily 

 be seen that the aperture of the Nicol will limit the illumination in the object 

 plane. This limitation is avoided by introducing two more collective lenses 

 between the first lens and the object. They are arranged so as to have a common 

 focus near which is placed the polarising Nicol. With this arrangement only a 

 small Nicol is necessary for transmitting all the rays. The extent of the illumina- 

 tion in the object is, under these circumstances, almost independent of the aperture 

 of the Nicol, depending only on the aperture of the lenses. Using this arrange- 

 ment, it is easy to project experiments illustrating the polarisation of refracted 

 light, double refraction by a calcite prism, production of axial images — inter- 

 ference phenomena— and many others. The small size of the Nicols allows the 

 apparatus to be placed on the market at a comparatively low price. 



(3) In Mikro 205, there is described a useful method of projecting spectrum 

 experiments, including a neat device for showing complementary colours and their 

 combination. 



H. Thirkill. 



