1 88 OPTICAL PROJECTION 



siderably, being however chiefly valuable for the reason here 

 explained. Such a length of tube also cuts off some of the 

 outer and most indistinct portion of the field. The farther 

 the lens is placed from the objective, the more the image is 

 amplified, the more it is contracted or cut off at the margin, 

 and the less the focus of the objective is altered from what it 

 would be if used alone. We may therefore use the amplifier 

 nearer the objective than its best ' correction ' point, for the 

 sake of a larger field ; or farther from it, for the sake of more 

 amplification ; in either case at a slight sacrifice of the best 

 performance of the objective. 



Practically I find the most useful range of adjustment 

 to be an objective-fitting and amplifier-tube which will allow 

 the amplifier to be placed four inches from the screw-threads, 

 or to be pushed in as close as two inches. The last will give 

 as large a field as the objective alone ; at four inches some of 

 the outer portion of the image is cut off, but the rest is more 

 amplified. Now and then it is useful to employ quite a long 

 (six-inch) amplifier-tube, and to use a high-power lens there. 

 The amplification then is very great; but only the central 

 portion of the image reaches the end of such a long tube, 

 which must be carefully guarded against ' flare ' by being 

 lined with black velvet. 



Amplification by Eye-pieces. Another way of ampli- 

 fying the original image, is to employ an eye-piece, at the end 

 of a tube-fitting of the ordinary length, instead of the short 

 fitting just described. Optically, this method is more perfect 

 than the other, and for photography far preferable, a sharper 

 image being obtained, and a flatter field, with a sharp circular 

 black margin to the disc on the screen. Practically, for 

 ordinary purposes, an eye-piece entails more trouble ; but 

 undoubtedly it gives the finest projections. 



Zeiss's well-known projection eye-pieces have lately made 

 this method familiar, and need no recommendation for photo- 

 graphic purposes ; but they are not suitable for ordinary screen 



