HENRY M. SAYERS 171 



The condenser must be of short focus in order to take in a large 

 cone of hght from the radiant. The Nelson condenser mentioned has 

 a working distance of about one-and-a-half inches. This is too short 

 for an arc of even ten amperes, but with a Nernst or half-watt 

 lamp up to 100 candle power the heat will not injure it. The Pointo- 

 lite lamp of 100 candle power has a bulb which is just too large for 

 such a condenser to focus at the required distance. 



Another arrangement is to set up a screen with an aperture of the 

 required size, say seven-eighths inch to one inch, which may con- 

 veniently be an iris, at the required distance and to throw on that 

 aperture a magnified image of the radiant. The image formed by the 

 objective on the object will then be a reduced one of the radiant. This 

 arrangement takes up a good deal of room. Thus if the aperture is 

 one inch in diameter and the radiant quarter-inch diameter, and a 

 lens of four inches focal length is used, the total distance from the 

 microscope body will be from 32 to 36 inches, which is awkward 

 for making the adjustments, attention to the arc, etc. It is doubtful 

 whether a lens of four inches focal length could be safely used with an 

 arc giving a quarter-inch diameter crater. A six-inch or eight-inch 

 focus would probably be required and proportionately more distance 

 occupied. It follows that there is not really much advantage in using 

 radiants larger than those which permit of the use of lenses of about 

 two inches focus. More light is produced, but no more is utilised. 



A third method is to present to the objective a virtual image of 

 the radiant, i.e., to use an auxiliary lens as a simple magnifier, the 

 objective taking the place of the eye. The focal length difficulty comes 

 in again, as the lens must be closer to the radiant than its focal length, 

 A lens combination with its equivalent plane well in front of it, so 

 that the working distance from the radiant is greater than the focal 

 length, gets over this. Such a combination which I have used with 

 success is a Nelson condenser with a flint concave between it and the 

 microscope. The combination is really a microscope of the Brucke 

 type. The concave is placed close up to the aperture of the vertical 

 illuminator, and focussed by moving the radiant to or from it. As the 

 radiant and condensing combination are both within a few inches of 

 the microscope body, adjustments are easily made while observing the 

 object. The image given by the objective is a real imiage of the radiant. 

 The magnification may easily be ten times. 



Whatever arrangement is used there should be provision for inter- 

 posing a ground glass or hght filter in the path of the beam. For 

 metallography a light filter is not needed for securing contrast as in 

 stained specimens photographed by transmitted hght, but for cutting 

 out the chromatic residuals given by even the best objectives. The 

 sharpest visual focussing on the camera screen without a filter fails 

 to give an equally sharp negative. A green filter, such as the F line 

 filter, or a malachite green gives sharper results without a great in- 

 crease in exposure. 



With either of the two first named auxiliary arrangements a glass 

 micrometer can be placed in the focal plane which is the virtual 

 radiant and the scale image focussed on the specimen can be photo- 

 graphed at the same time. This is equivalent to an eye-piece micro- 

 meter. Its size on the camera screen is a measure of the magnification 



