1082 Transactions of the American Institute. 



tion or motion of the sitter or object to the extent of .1000 of an 

 inch would not be seen in ordinary photography ; but any one will 

 readily understand that when such motion is magnified, for instance, 

 1000 times by the objective of the microscope, we then have, instead 

 of .1000 of an inch, one full inch of motion, shown in the resulting 

 •image, which would make it far from satisfactory. And when we 

 consider the minute pencil of light used in working with a lens, the 

 diameter of which does not exceed the one-twentieth of an inch, and 

 a focal length of one-fifteenth or one-thirtieth of an inch, and that 

 with such conditions the relation of light and shade must be main- 

 tained on objects invisible to the naked eye, it will at once be apparent 

 that in using either direct or reflected solar light, the motion of the 

 earth and consequent continual change in the angle of the incident 

 solar ray must not be disregarded. 



To constantly change the position of the apparatus to compensate 

 for this, requires a most delicate and nicely adjusted mechanical 

 device, or we must maintain the impinging light in a constant direc- 

 tion by the use of a heliostat. But v/hen the atmosphere is so 

 charged with vapor as to obstruct the direct rays of the sun, our 

 work must stop ; difi'used light illuminating an object placed in the 

 focus of a microscopic objective of high power, produces an image 

 far too weak for recognition by nitrate of silver. 



With a good artificial light, constant in direction, and amount of 

 actinic force, and at the same time easy to control, we may work at 

 night when the vibration from streets and manufactories have 

 ceased. 



Up to the present time science has given us but few methods for pro- 

 ducing light, at all adequate to the ordinary purposes of photography. 



All successful experiments thus far seem to have been conducted 

 by the use of electricity, burning the metal magnesium, and the com- 

 bination of the two gases, oxygen and hj'drogen, and in tlie presence 

 of zireonia^aud lime, all of which give off more or less actinic force 

 when raised to an incandescent heat. Of these, electricity with 

 carbon poncila is by far the jnost energetic. 



The instantaneous flash of no other light will produce eflfects of 

 equal intensity. Its entire volume seems to be made up of concen- 

 trated actinic power, which justifies us in placing it first of all 

 artificial lights. Its use has been limited by the cost of production, 

 and inconstant amount of light, owing, in a great measure, to imper- 

 fect methods of application. 



