106 



I accordingly directed my attention towards working with direct 

 sunlight and, after trying various methods of illumination, I adopted 

 the following with high powers, such as the l, Jg' ^^'^ tV '^^^^ °^" 

 jectives. 



I placed the microscope as before in relation to the camera, and 

 then set the whole perpendicularly with the sun, the camera being 

 nearest the sun. Then I plac^ a mirror (which I find should be 

 without many blemishes, since otherwise colored rings, &c. are apt to 

 appear on the focussing plate) in front of the microscope, and arranged 

 it at such a height and angle that the sunlight was thrown directly 

 through the microscope upon the focussing slide of the camera. I 

 condensed the light upon the object on the stage of the microscope by 

 means of an achromatic condenser, and still further by means of a 

 large bull's eye condenser, placed between the mirror, before spoken of, 

 and the achromatic condenser, and so arranged that the light was 

 brought to a focus upon or near the open end of the achromatic con- 

 denser. This was ascertained by simply holding a piece of glass 

 over the open end and bringing the light to a focus upon it, care 

 being taken that this condensed light was made to pass in a direct line 

 through the achromatic condenser upon the object, thence through 

 the objective used, and thence directly upon the focussing slide. 

 Then by rotating the screw of the achromatic condenser, more or 

 less light could be thrown upon the object, according to the objective 

 used. 



For low powers of course no such condensation of the light is neces- 

 sary. We may dispense with the achromatic and bull's-eye condensers 

 and the mirror, and condense the direct sunlight upon the object by 

 the small mirror attached to the microscope. 



Being soon satisfied that there must be some loss in the details, by 

 going through the two processes of obtaining a negative, and then 

 printing from it, I then turned my attention to direct positives on 

 glass or ambrotypes, such as are now before you. These ambrotypes 

 require only one half the time of exposure in the camera that photo- 

 graphs do, which is also an advantage when working with high powers, 

 when the focussing slide is sometimes placed at a distance of five feet 

 from the objective, by which means the light becomes diffused over a 

 large surface, attended with an ecj[uivalent loss of intensity. 



I have, on some occasions, when I wished to bring out, with sharp- 

 ness of definition, some of the indistinct fines or dots on Diatoms, 

 placed a diaphragm, having a very small aperture immediately be- 

 hind the objective in the tube of the microscope. By so doing, of 

 course, much light is lost, but here again, the extreme sensitiveness of 

 the ambrotype process renders it much superior to the common 

 photograph. 



It is generally stated, in all articles on micro-photography, that the 



