THE PROJECTION MICROSCOPE 187 



slides into the body-tube carrying the objective, and must be 

 guarded against any ' flare ' or internal reflection from its 

 sides. The effect of such a lens is to lengthen the focus of 

 any objective in any given position ; hence its use also 

 increases the working distance. For photographic purposes, 

 the amplifier should be corrected; but for ordinary projection 

 a simple lens is sufficient. It is convenient to have a set of 

 amplifiers of various foci, which give a great range of powers, 

 and of working distances from the screen, enabling very similar 

 results to be obtained under different conditions. 



The use of an amplifier has another considerable advantage. 

 Ordinary micro -objectives are corrected to a focal length of 

 about 10 inches in England, and 6 or 7 inches on the Con- 

 tinent. If such lenses are used to project an image, say, 

 25 feet away, besides the mere greater magnification of any 

 error, it is obvious that their ' corrections ' can no longer be 

 accurate. It is really wonderful that some lenses should 

 perform, under such trying conditions, so well as they do. 

 But now suppose some given lens is focussed on a slide, so as 

 to project an image at its own proper distance of 6 inches. 

 Our screen however is, we will say, 25 feet away ; a distance 

 I have found to give on the average perhaps the best range of 

 general results, though shorter and longer are better for some 

 purposes. We can select a concave amplifier of a focus which, 

 placed in a certain position, will bring the image to a focus 

 on the screen without altering the focus on the object, thus 

 maintaining the 'corrections ' of the objective at nearly their 

 proper value. The late Colonel W. Woodward, U.S., was, I 

 believe, the first to point out this fact, which is of great use 

 in photo -micrography. 



Any concave lens can be placed somewhere so as to focus 

 a 6-inch or 10-inch focus on the screen, at any longer distance ; 

 but practically we are limited to a certain range. I have 

 found generally useful quite a low power, which does this at 

 about four inches behind the objective, and amplifies con- 



