ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 675 



the first illuminating lens to be utilized, a second condition must be 

 fulfilled, viz. that the image of the source of light which falls on 

 the object must not be larger tban the object itself. Since the source 

 of light aud its image are as the two focal lengths it is obvious that 

 these conditions can only be strictly fulfilled with very low powers 

 and under very favourable conditions. With higher powers the 

 greater part of the light is lost for this reason, that the intensity of 

 the light with the higher powers diminishes not with the second, but 

 approximately with the third power of the amplification. 



The greater part of the light from the lamp does not fall on the 

 first illuminating lens. In order to utilize as much of this portion 

 as is possible the attempt has been made to concentrate by means of 

 large concave mirrors the light which is lost on the side opposite to 

 the condensing lenses. The mirror — which should be concave — 

 must have the flame in its centre of curvature, the image of the 

 flame, therefore, coinciding with the flame itself. As this is trans- 

 parent, only a small portion is lost by absorption, and the part 

 that is utilized follows the same direction as the other rays. 

 This condition is absolutely necessary, in order to avoid light- 

 nodes in the illuminating cone produced by two different con- 

 verging rays, whereby the clearness of the image is materially 

 affected. It is thus evident that with all ordinary flames only the seg- 

 ment of a small circular surface is utilized. The flat flames, the 

 narrow edge of which is used (as for example in the Sciopticon), give 

 the best results. On account of the too great extent of the illumi- 

 nating surface, lighthouse lamps, which consist of a number of 

 concentric wicks, only yield a very moderate result, notwithstanding 

 the quantity and intensity of their light. Fresnel's ring-lenses are 

 also unsuitable. Illuminating lenses of the smallest dimensions and 

 the largest aperture angle (as near as the temperature of the flame 

 will allow) give the best results. It is also advisable to insert a 

 movable lens between the object and the illuminating system, in 

 order to regulate the convergence of the light according to the 

 requirements of the objective employed. To obtain a perfectly 

 uniform illumination of the image-surface it is further necessary that 

 it should not be the image of the source of light produced by the 

 illuminating lens that falls on the object, but a neighbouring aberra- 

 tion-circle, in which the light is uniformly distributed. (Petzval has 

 already drawn attention to this.) 



Besides lamplight the Drummond lime-light has been employed 

 very satisfactorily, and after many experiments Dr. Schroder considers 

 it the best on account of the small and intensely illuminating surface 

 of the lime and its pleasant light. In spite of its intensity, the 

 magnesium-light gives no satisfactory result, because it does not 

 burn steadily, and even when a ventilator is employed, the lenses are 

 covered with the burnt magnesium. The electric light is excellent 

 on account of its large intensity in a small space, but its unsteadiness 

 is objectionable. The Jablochkow candle is most suitable, notwith- 

 standing its small intensity, if a uniform height can be maintained. 

 The incandescent light is too small in intensity, and too oblong. 



