666 Transactions of the Society. 



note-book a memorandum referring to another amplifier, I wrote 

 him that it had a virtual focus of about 12-5 centimetres, whereas 

 its focus is really somewhat more than 16. 



I do not, however, doubt in the least that excellent results may 

 be obtained with a properly constructed amplifier of the focal length 

 he has adopted, and can express no opinion as to how far he has 

 succeeded in this direction, because I have not yet seen one of those 

 he is making. But I must point out that it is not enough to have 

 a properly constructed amplifier, and to screw it at the end of the 

 draw-tube ; it is also of prime importance that the proper position 

 of the amplifier should be found for the distance selected. If this 

 is neglected, very unsatisfactory results may be obtained with 

 excellent instruments. 



I must add, that the plan of using a negative lens behind the 

 objective, for the purpose of increasing the size and flatness of the 

 image formed by a solar or gas Microscope on the screen, is quite 

 an old device, as is also the plan of using a negative lens between 

 the eye-piece and objective (preferably at the end of the draw-tube) 

 for the purpose of increasing the magnifying power of the Micro- 

 scope. I was, however, the first to point out the advantages of 

 employing a suitable achromatic concave in place of the eye-piece 

 of the Microscope for the purposes of photo-micrography. I had 

 already published a preliminary notice of this method in 1865,* 

 and described it in more detail in 1866.t Since then I have 

 repeatedly, in published articles, in private letters, and by word of 

 mouth insisted upon this point, until it has become so familiar that 

 some of my correspondents imagine it to be their own contrivance. 

 I have not hitherto, however, nor has any other microscopist, to my 

 knowledge, commented upon the important application of the achro- 

 matic concave described above, in which, by giving it suitable focal 

 length and position, it is used to project the image upon the screen 

 while the object-glass is in precisely the position with regard to the 

 object that it would occupy in ordinary work, and the course of 

 the rays through it quite the same. 



II. The second point to whicli I wish to call attention relates 

 to the obliquity of the illuminating pencil necessary to resolve 

 difiicult lined test objects, such as Ampliij^leura pellucida mounted 

 in Canada balsam, by objectives of adequate aperture. Until a 

 comparatively recent period many respectable microscopists still 

 clung with affection to the singular theoretical error that the angle 

 of aperture of immersion objectives, as measured in a medium 

 (such as Canada balsam) of about the index of refraction of crown 



* Circular No. 6, War Department, Surgeon-General's Office, Washington, 

 1865, p. 149. 



t 'American Journal of Science and Arts,' vol. xlii. (1866) p. 189; also 

 ' Quarterly Journal of Microscopical Science,' vol. vi. (1866) p. 166. 



