Photography in the Daguerreotype Process, 385 



investigation of the question Mr. Knight's apparatus will 

 be most valuable to tne optician, as it will afford him the 

 means of studying the phaanomenon with mathematical accu- 

 »acy. 



Mr. Knight's apparatus consists in a frame having two 

 grooves ; one vertical, in which he places the ground glass, 

 and the other forming an angle with the first destined to 

 receive the plate; the planes of the grooves intersect each 

 other in the middle. After having set the focus upon the 

 ground glass, this last is removed, and the plate is placed 

 in the inclined groove. Now if a newspaper or any large 

 printed sheet is put before the camera, the image will be 

 represented on the inclined plate ; and it is obvious in its 

 inclination the various points of the plate will meet a dif- 

 ferent focus ; the centre of the plate will coincide with the 

 visual focus by its inclination. It will in one direction meet 

 the photogenic focus at a point more or less distant from the 

 centre, if the photogenic focus is shorter than the visual focus, 

 and in the other direction if it is longer. The frame is fur- 

 nished with a scale of division, having the zero in the centre. 

 When the image is represented on the Daguerreotype, by 

 applying against it anotlier moveable scale of division similar 

 to the other, the operator can find what is the division above 

 or under zero at which the image seems best defined ; and 

 after having removed from the camera the experiment frame, 

 and set the focus as usual on the ground glass, he has only to 

 move the tube of the object-glass by means of the rack and 

 pinion, and to push it in or out, a space corresponding with 

 the division of the scale indicating the deviation of the true 

 photogenic focus : the tube of the object-glass is for that pur- 

 pose marked with the same scale of division. 



In order to enable the members of the Association to judge 

 of the merit of Mr. Knight's invention, I have had his appa- 

 ratus applied to a small camera with which I made my expe- 

 riment. By exhibiting at the same time Mr. Knight's method 

 and my own, a comparison of the two may be made, and 

 they will be both better understood. 



Before concluding, I shall call the attention of all persons 

 conversant with optics to the singular fact I have observed 

 respecting the constant variation of the two foci. I have not 

 been able yet to find its cause, and I leave its investigation to 

 more competent persons. I hope at the next meeting of the 

 Association we shall know more on the subject. 



Phil. Mag, S. 3. Vol. 35, No. 237. Nw. 1849. 2 C 



