290 TABLES FOE CORRECTING ERRORS IN CAMERA DRAWINGS. 



The above rules apply equally to photomicrography when the 

 projection distance is 10 inches. The percentages in the 3rd 

 column of Table I., and the errors in the 2nd and 4th columns 

 of Table II., may be found for projection distances other than 

 10 inches, as they are inversely proportional ; thus at projection 

 distances of 20 inches the percentages and errors will be half, and 

 of 5 inches double the amounts given in the following tables. 



If a camera of the form designed by Mr G. Burch and ex- 

 hibited here in 1878, now known as Abbe's camera, be used the 

 distance of the paper from the mirror must be 10 inches minus 

 the distance of the mirror from the centre of the eye lens. 



Example. — The image of '01 inch projected by Dr. Beale's 

 camera on a rule distant 10 inches from the eye lens measures 

 8 inches. What is the true magnifying power ? 



Ans. : — 800 less 5 per cent. = 760 diameters. 



II 



Diameters. 



