14 PROPERTIES OF LENSES. 



so opticians overcome the difficulty by combining 

 several. 



Before leaving fchis subject we should mention 

 thai in photography errors frequently arise, espe- 

 cially when using low powers, owing to the differ- 

 ence between the visual and chemical loci. 



A point o[' white light may appear perfectly dis- 

 tinct at K Pig. 9, when surrounded by a violet areola, 

 and if a photograph be taken it will be quite indis- 

 tinct ; while at V nearer the lens the image may not 

 appear so distinct, but will be surrounded by a red 

 areola, and it a photograph be taken at this point it 

 will be found perfect, as the violet, rays are the most 



chemically aotive. 



This is owing to the different refrangibility o\' the 

 red and violet rays, the latter coming to a focus 

 sooner than the former; when photographing, the 

 point V should be chosen when the object is sur- 

 rounded by a red areola, this being the most ac- 

 curate for the violet rays. 



Microscopic objectives have of late years arrived, 

 at such perfection that it seems impossible to im- 

 prove them. 



They are designated from four inch up to one 

 fiftieth ol' an inch, according to their magnifying 

 power: for example, a one inch objective is supposed 

 lo magnify as much as a single lens of one inch 

 focus, although its own focal length may be different. 



This rule is only approximately correct, as glasses 

 oi' the same designation by different makers vary in 

 magnifying power. 



