MICROSCOPIC ILLUSTRATIONS. 253 



9. This illumination consists in causing the light to fall 

 upon the object obliquely ; moreover, the line of the said 

 obliquity must traverse the system of lines intended to be 

 brought out nearly at right angles, though there are some 

 anomalies in this respect; and the intensity of the light 

 must be such as to produce a maximum of blackness or 

 darkness in the lines, for which purpose a faint illumination, 

 whether the light is artificial or natural, invariably answers 

 best. All condensations by concave mirrors and lenses are 

 hurtful, and cause the lines to assume a faint nebulous 

 appearance, as if drawn with a pencil instead of a pen and 

 ink. There is also another false phenomenon produced, by 

 condensing light on the plane of the object ; for example, 

 on the lozenges, on the scales of the podura, which I have 

 sometimes seen exhibited by such illumination, just in the 

 opposite way to which they ought to be shewn, namely, 

 with the spaces between the lines dark, and the lines them- 

 selves faint ; so that the scale somewhat resembled a sam- 

 ple of the moss hypnum. I have, moreover, seen this 

 style of vision very much admired by connoisseurs. 



10. There is an infinite number of ways in which lined 

 objects may be illuminated, so as to fulfil the aforesaid 

 indications more or less perfectly ; but I am of opinion 

 that there is nothing equal to a wax taper placed behind 

 the stage, without any other apparatus whatever, both from 

 its simplicity and its very manageable nature. It is some- 

 times advisable to put the slider-holder on the wrong way, 

 that is, on the opposite side of the stage to that on which 

 it is customary to fix it, so that there may be no aperture 

 of any description behind the object, which is thus left 

 perfectly exposed to the most oblique ray. This method, 

 moreover, allows the candle to be brought extremely near 



