2i OPTICAL PROJECTION 



up. For much long-focus work, an extra condenser (also of 

 longer focus), gives the best result. 



For all usual exhibition purposes the ordinary double-piano, 

 with the lenses in contact, answers very well ; and it is only 

 for sharp detail in the best class of photographic slides, or 

 when it is desirable to utilise the field as far as possible up to 

 the very edge of the disc as in square or cushion slides 

 that such optical refinements acquire importance. 



11, Practical Points in Condensers. Condensers are 

 usually made of crown glass, density about 1*516. This 

 answers perfectly for ordinary purposes ; but for the highest 

 glass of modern exhibitions, which have to cover large screens 

 of twenty to thirty feet diameter, sometimes at great distances, 

 it is objectionable, as the green colour absorbs light which 

 can ill be spared in such circumstances. Chance's ' optical ' 

 flints, of course, leave nothing to be desired in this respect, 

 but condensers so made are very expensive ; in some cases, 

 however, they are worth while, as the glass is both more 

 colourless, and the lenses are thinner for the same focus. A 

 more common flint would answer practical purposes, and it is 

 desirable that some colourless glass should be introduced if 

 possible, rather than green crown. This need not make them 

 so very much more expensive ; for while ' optical ' flint must 

 be homogeneous, and is usually wanted dense, for a condenser, 

 a perceptible amount of stria is of little practical importance, 

 and density is not required, only colourlessness. It is utterly 

 useless to pay the cost of optical perfection in a condenser 

 for any ordinary purposes. A perceptible bubble in the lens 

 next the slide, however, would be a defect of importance, 

 probably showing as a black spot. It is very likely that many 

 purchasers would reject a colourless lens with perceptible striae, 

 rather than a crown lens which showed none ; nevertheless 

 there can be no question that the first would be the better 

 condenser, unless the striae were excessive. 



The point most commercial condensers chiefly fail in, 



