152 Transactions of the 



makes a remark which Dr. Pigott interprets as underrating this 

 mode of representation. After describing the arrangement of the 

 hemispherical elevation on Pleurosigma quadratum and P. angu- 

 lafum, he says — " Hence, under the illusion of the common methods 

 of illumination, which deal with shadows only, and under deep 

 powers, the markmgs of those diatoms are described and figured as 

 hexagons, with the sides and centres Hght and dark, or vice versa, 

 and PHOTOGRAPHY stands by as an attesting witness." 



Dr. Pigott, in his paper published last November,* quotes this 

 sentence and expresses himself stiU more strongly. Speaking of 

 the beading he describes on the Podura scale, he says : — " It would 

 seem the actinic glasses fail at present to perfect these views. 

 Illumination is a vexed question for the photographer, who has 

 never yet succeeded in displaying the spherules of the Ampliipleura 

 joeUucida, which so many Enghsh observers have seen microscopi- 

 cally. Unless, therefore, the defining power of photography at least 

 equals the human eye armed with the microscope, no rehable argu- 

 ment can be drawn from its failure or approximate revelations. 

 Indeed, the President notices that photography indicated hexagon 

 forms as the correct appearance for the diatom headings, which we 

 now know is false." 



I do not think the President's remarks were intended to express 

 the notion they are here quoted to support, and I beheve I shall be 

 able to show that the paragraph from Dr. Pigott 's paper is inaccurate 

 in every particular. Indeed I am at a loss to understand how some 

 of the misconceptions it contains could have arisen at this late day. 



In my first essay on photo-micrography, published in the 

 ' American Journal of Science and Arts,' vol. xlii., September, 

 1866, I expressed the opinion that photography was "adequate to 

 the satisfactory representation of all microscopical objects that do 

 not depend for their value on colours." Subsequent experience has 

 satisfied me of the substantial justice of this opinion. 



In examining objects with the microscope with white light, 

 both colour and form produce impressions upon the eye, and of 

 course it must be fi'eely granted that photography cannot be counted 

 upon as a means of representing the colour phenomena. If, how- 

 ever, the microscope be illuminated by monochromatic hght the 

 colour disappears and form alone is perceived, the object appearing 

 black or shaded on a ground of the colour employed. A trial wUl 

 convince anyone that so far from impairing definition, this method 

 considerably improves it, and for mere purposes of observation it 

 matters little what kind of monochromatic light is employed. I find 

 blue or violet light most agreeable to the eye, and these are also 

 the colours suitable for photography ; but for mere observation any 

 other colour will answer. Such light may be obtained from the 

 * ' Monthly Microscopical Journal,' vol. iv., p. 254. 



