244 SUMMARY OF CURRENT RESEARCHES RELATING TO 
tlie plane mirror, in no way affects tlie final result. Accordingly, when- 
ever the source of light can be regarded as limitless for a given amplitude 
of illumination, no apparatus, concave mirror or condenser, will produce 
a different effect to that obtained by the use of the plane mirror alone. 
Fig. 23 . 
This will always be the case when objectives with total angle of aperture 
less than 30° (No. 1 of Nachet., A of Zeiss, and lower numbers) are used 
with the Microscope placed before a window widely open on a horizon 
sufficiently vast, and with the sky free from clouds. 
(2) The source of light is limited. This is usually the case, and is 
caused either by the source being really limited in itself, or by the 
plane mirror which transmits it being too small to receive all the rays 
proceeding from the object A condenser is therefore necessary in 
order to cause all the rays emanating from the object to meet the 
source of light. The concave mirror is a simple but imperfect con- 
denser, which suffices for low-power objectives. Its angular aperture is 
about 30°. 
Tor high'power objectives a condenser is required. This forms at a 
given point a real and reduced image of the source, and the position of 
the object must be on this image. Here the intensity of the light is at 
a maximum, and since the object coincides with a source of light, all 
phenomena of diffraction are eliminated. 
As regards the achromatism of the illumination, it is sufficient that 
