674 SUMMARY OF CURRENT RESEARCHES RELATING TO 



40° or even to 60°, whereby, under equal circumstances, a 36-tiines 

 larger surface can be viewed, and by 100 and more spectators. 

 It is also very useful for pointing out to beginners particular parts of 

 the object, in the same way as a drawing would be explained. The 

 observation of the projected image requires no especial practice, as in 

 the compound Microscope ; and finally the image can be easily drawn 

 or even photographed. 



Notwithstanding all these advantages, however, these instruments 

 — called by Professor Petzval the " chef-d'oeuvre of optical art" — have 

 hitherto been very hardly treated. Usually the lenses of a compound, 

 Microscope (often most unsuitable) were employed, and illuminating 

 lenses with surfaces exactly convex, thus constituting a very indifferent 

 instrument. The necessity of employing a heliostat, and the difficulty 

 of always obtaining sunlight at the required moment, gave an impulse 

 to the construction of the so-called lantern Microscope used only with 

 artificial light, and in the last century Adams was celebrated for such 

 instruments, which could be used in several ways, as simple, compound 

 or lantern Microscopes. Their performance was best as simple, mode- 

 rately so as compound, and very inefficiently as lantern Microscopes. 

 Much later, when achromatic objectives were introduced, 

 Chevalier in Paris and Duboscque constructed much more complete 

 instruments, and in modern times Foucault invented the excellent 

 photo-electric projection-Microscope. 



At first sight nothing seems simpler than to construct a good 

 lantern Microscope since we have only to replace sunlight by lamp- 

 light. This is, however, not the case, for on further consideration it 

 will be found that the conditions which are so favourable with sun- 

 light cannot be maintained with any artificial light — we can only 

 approximate to them. The intensity of all artificial illumination, 

 even the strongest electric light, is considerably less than that of the 

 sun ; besides, all strong lights have far too large an illuminating 

 surface to give distinct images with many fine details. The earlier 

 lantern Microscopes had the worst possible illumination, for good 

 oil-lamps did not then exist. If petroleum or gas lamps be used, it 

 will soon be found that the magnitude of the flame in no way 

 heightens the effect; although the image surface may appear to be 

 more brightly illuminated, the contrast between the light and dark 

 parts will be less — the absolute intensity is greater, but the relative 

 smaller. If we follow the course of the illuminating rays it will 

 be seen that the flame limits light diverging in all directions. 

 Divergent light cannot, however, be employed for the illumination of 

 an object, but we must always have convergent light. The source of 

 light is therefore placed in the first focal point of a convex illumi- 

 nating lens and the object in the second. The nearer a lens of given 

 diameter is to the source of light, the greater will be the aperture- 

 angle of the illumination ; the greater the quantity of light utilized 

 the further off will be the second focal point and the less the con- 

 vergence of the rays upon the object. The convergence of these 

 rays must, however, correspond with the final convergence of those 

 which limit the field of view, and therefore, for all the rays falling on 



