OBLIQUE ILLUMINATION. 247 



are tempted to assume an error of half a wave-length in the deter- 

 mination of the difference of phase. Indeed, Valentin 1 gives, as 

 the thickness of the layer of air for the first dark ring, one entire 

 wave-length, that is, exactly double the value in the above formula ; 

 and similarly for the other parts and also for Newton's colours. 

 But the figures given by Valentin are erroneous for reflexion in 

 layers of air, glass, or liquids, 2 and there is at present no reason for 

 assuming any other proportion with organic membranes. 3 We 

 should rather suspect that the observations of Floegel on Pleuro- 

 sigma were influenced by the reflexion of light on the walls of the 

 chambers. It appears to us d priori to be questionable whether a 

 membrane of such peculiar structure will act as a homogeneous 

 substance. 



IV. 



OBLIQUE ILLUMINATION. 



WE have already stated that a cone of light incident obliquely 

 may act more favourably with regard to the aberration of the 

 objective than an axial one that is, on the supposition that just 

 those inclinations are represented for which the instrument is 

 most perfectly aplanatic. We will entirely disregard here this 

 consequence of oblique illumination, and assume we have a 

 perfectly aplanatic Microscope, in which the oblique position of 

 the incident cone of light influences the intensity of light, but 

 not the distinctness of the image. There still remains to be 

 discussed the question, how oblique, as opposed to axial, illumina- 

 tion acts upon the distribution of light in the microscopic image 

 if the object is a substance composed of delicate layers or a 

 membrane with slight elevations or depressions. Since both 

 these cases may, with reference to their total effect, be. reduced to 

 the one that corresponding portions of surface of the field of 



1 Valentin: " Untersuch. d. Pflanzen- u. Thiergewebe im polar. Licht" 

 (1861), p. 120. 



2 Cf. Wilde, in Poggendorff's " Annalen," Bd. Ixxx. p. 407; Bd. Ixxxiii. 

 p. 18. 



3 Cf . moreover Quincke : " Ueber die Aenderung der Phase bei der Reflexion 

 der Lichtwellen." Poggendorff's " Annalen," Bd. cxlii. (1871), p. 192. 



