24 THE BOOK OF THE LANTERN. 



L R, after refraction by lens A, diverges into a prismatic 

 beam; this falls upon different parts of lens B, which, 

 while acting upon the two extremes, the red and the 

 violet, in contrary directions to A, causes them to emerge 

 parallel, the condition of achromatism, when they converge 

 to the conjugate focus f, about 9 in. removed from B. The 

 spherical aberration is reduced to a minimum by the forms 

 of the lenses employed, i.e., ray Lr, refracted by the central 

 portions of the lenses, meets the axis at the same point f, 

 as the marginal rays, or nearly so. I have decided upon a 

 4-in. (effective) diameter condenser, since it fully illumi- 

 nates the corners of a 2|-in. square slide. Of course, a 

 circular slide of 3 in. only requires a 3J-in. diameter con- 

 denser, of proportionately shorter focal length. I need 

 hardly say that the glass composing this condenser has 

 been selected with especial care. It is perfectly limpid, or 

 colourless, and will remain so ; it is free from stricB and air 

 bubbles, and has a perfect polish. In fact, it is Chance's 

 best glass ; the only drawback being its cost." 



Mr. Dallmeyer goes on to remark that the defects in the 

 glass of a lantern condenser, are of far more importance than 

 similar defects in the objective, so far as purity and quality 

 of the illuminated disc are concerned. He refers to such 

 defects as scratches, air-bubbles, and the like. In the case 

 of an air-bubble in the objective lens, it is really of no 

 moment whatever. I have known of a photographic lens, 

 otherwise of splendid quality, being rejected because of 

 a tiny air-bubble near its margin, the purchaser being quite 

 content to exchange it for a far inferior lens without such 

 an insignificant blemish. Such a bubble would have no 



