58 Oblique Illumination for the Binocular. 



illumination the practised observer is well aware that it is desirable 

 to exclude as many as possible of the direct rays which suffer little 

 or no refraction in their course upwards, but tend to produce an 

 uncomfortable and confusing glare, and to accomplish this, the 

 mirror is generally thrown a little out of axis, so as to give a slight 

 degree of obliquity to these portions of the light, as well as to cast 

 the shadow more or less to one side of the object. Thus, although 

 we speak of direct illumination, it is not so in reality ; a slight 

 degree of obliquity is found to be a practical necessity, and I have 

 been endeavouring ever since the introduction of the binocular to 

 find some means of obtaining a more satisfactory arrangement for 

 accomplishing this, and have at length been rewarded with sufficient 

 success to warrant its being offered to the attention of others. It 

 has always appeared that as the outside edge of the picture of a 

 flame is found to be the most effective, there is something more than 

 the mere obliquity of the rays that is the desideratum, and I have 

 been inclined to attribute it to inflection, or diffraction, and this 

 idea has been kept in view throughout. 



The first point has been to stop out all the useless central rays. 

 The next to throw up a good body of light in a suitable direction to 

 produce the necessary shade and shadows, and then to let in a sufficient 

 amount of light from another point in order to render the shadows 

 transparent and thus enable us to distinguish the detail within 

 them. As a foundation to commence upon, a hemispherical lens 

 of about one inch in diameter was found to be most advantageous, 

 but it was also found that placing the diaphragm in contact with 

 the upper flat surface of the lens, as recommended by the late Kev. J. 

 B. Eeade, did not produce the same effect (which I attribute to dif- 

 fraction) as placing it beneath the convex surface and at a very slight 

 distance below it. After innumerable trials it was found that the 

 FlG 3 form (accurately ob- 



tained by tracing) re- 

 presented in Fig. 3 

 gave the best results. 

 This is fixed by the 

 tongue A being bent 

 up, and then inserted 

 into the doubled end of 

 the flat ring at B, Fig. 4. 

 This ring is left as a 

 fixture within the set- 

 ting of the lens, and so 

 admits of the stop being removed at pleasure or changed for any 

 other form. The stop is cut out with a pan of scissors from sheet 

 tin about the thickness of thin writing paper, and is then curved at 

 the transverse edge to something less than the curvature of the lens, 



