1S90.] MICIIOSCOPICAL JOURNAL. 261 



as the paper may be purchased ready sensitized, at very trifling cost, and 

 it requires no skill or experience in the using. It is merely necessary 

 to expose to bright sunlight until sufliciently printed (a few experiments 

 \\ill determine this), antl then to wash in several changes of water ; the 

 result being a bright, permanent blue print upon a clear, white ground, 

 with excellent detail, excepting in the most delicate structures. 



The negatives made with the "■ Handy" camera are of a convenient 

 size for prmting lantern slides by contact. A print on glass is certainly 

 the most perfect of any that possibly can be made, and the importance 

 of this method of demonstration has long since been conceded. Gela- 

 tine plates coated on thin glass with special slow emulsions, are fur- 

 nished by several makers, and any microscopist can readily make his 

 own lantern slides with a little expenditure of time and patience. 



On a Mooted Matter in the Use of an Eye-Piece in Photo- 

 Micrograpliy. 



By a. CLIFFORD MERCER, M. D.. 



SYRACUSE, N. V. 



[Read at the Detroit meeting, American Society of Microscopibts, 1890] 



At last 3'ear's meeting of this society, the writer opposed the ordinary 

 use of the eye-piece in high power photo-micrography. He held that 

 a sensitive objective nicely adjusted for cover thickness and focussed 

 cannot suffer a change in position without having the nicety of its ad- 

 justment to some extent vitiated. The ordinary use of the eye-piece in 

 photo-micrography involves such a change in position, and therefore 

 vitiates to a corresponding degree the resulting image. 



Let us see how such a change is so involved. Rays of light leaving 

 a point in an object in focus and entering the microscope pass out of 

 the eye-piece divergent or parallel to enable the normal eye to focus 

 them on the retina. These rays must be divergent or parallel because 

 only such rays are focussed on the retina by a normal eye. Now, as 

 divergent or parallel rays cannot form a real image, the microscope 

 under the foregoing conditions does not project an image on a screen 

 held anywhere "above the eye-piece. To get an image above the eye- 

 piece the divergent or parallel rays are in ordinary practice made con- 

 vergent and, therefore, image-forming by focussing ; and it is this 

 procedure which changes the position of the objective. 



On the other hand. Dr. Blackham said, in I'eply, that when an object 

 is in focus for a normal eye looking through the microscope, a plane 

 can be found somewhere above the eye-piece in which a real image of 

 the object is formed. A second focussing is, therefore, unnecessary, 

 and the adjustment or theobjective is not disturbed. A few members 

 reported that they had secured photo-micrographs in this plane. Dr. 

 Blackham undertook to demonstrate the formation of the image bv 

 means of a solar microscope in an adjoining room. The prescribed 

 conditions seemed to be met, and, apparently, an image was formed on 

 a screen about ten inches from the eye-piece. 



Dr. Blackham's demonstration was accepted as satisfactory and con- 

 elusive. He was, however, unable to give, when requested, a diagram- 

 matic or theoretical explanation of the formation of the image. Another 

 member had puzzled over the apparent conflict between observation 



