MEMBRANES WITH SMALL DEPRESSIONS OR HOLES. 217 



adjustment to its plane will in every achromatic instrument give 

 a colourless microscopic image ; a higher or lower adjustment, on 

 the other hand, producing the well-known red and blue fringes. 

 Consequently, small hollow spaces, which enclose a medium of less 

 refractive power (as the lumina of bast-fibres, the nuclei of starch- 

 grains, &c.), if viewed with a chromatically under-corrected 

 Microscope, will appear colourless and bright with low adjustment, 

 and reddish and finally dark with higher adjustment. The true 

 mean focal adjustment to the centre of the hollow space lies, under 

 .all circumstances, between these two extremes, and gives, therefore, 

 if the dimensions are sufficiently small, a reddish image. 



II. 

 OBJECTS OF IEKEGULAE FOKM. 



1. MEMBRANES WITH SMALL DEPRESSIONS OR HOLES. 



IT is evident that small concave depressions or furrows, as 

 represented in Fig. 117, will act as concave lenses, and will 

 consequently give a virtual image of 

 the diaphragm on a suitable lowering |J| 

 of the body-tube. And indeed this ^ - 



virtual image must appear as distinct to 

 the naked eye as if viewed in the Microscope, notwithstanding its 

 minuteness. As soon, however, as the form of the depression 

 is essentially altered, if, for instance, it becomes prismatic or 

 cylindrical, and has a fiat base, the image will be lost to the naked 

 eye, while it remains perceptible in the Microscope by reason of 

 the difference in the angle of aperture. We may prove this by 

 spreading out a solution of common salt, sulphate of magnesia, &c., 

 upon the object-slide, and allowing it to dry in the air. In certain 

 places are formed large homogeneous lamellae or extended incrusta- 

 tions, with an infinite number of small porous holes or depressions, 

 cracks, furrows, &c., of the most different forms which are some- 

 times very beautiful on the addition of diluted alcohol. In most 

 of them the image of the diaphragm, window-frame, &c., appears 

 with tolerable sharpness if we lower the focus slightly below the 



