22 



are useful for objects too large for the field of the micro-objective. 

 A method I have employed with success in the photography of large 

 insects is to use a wide angle photographic lens with a camera of 

 extra long extension. My own has a triple extension specially made 

 for this purpose. Before leaving the subject of objectives, I might 

 mention that the higher powers, such as the T V in., are described as 

 oil-immersion lenses, from the fact of their requiring for the more 

 perfect exhibition of their resolving powers a stratum of oil between 

 the lens and the cover-glass of the object. Cedar- wood oil is 

 generally used for this purpose, the high refractive index of which 

 enables outside rays to be collected which would otherwise be lost. 

 It would be out of place in a paper of this scope to explain the 

 meaning of the term "numerical aperture" (n. a.). I will, however, 

 mention that, in effect, the higher the angle of aperture of an objec- 

 tive, the better is the definition in one plane of vision only, whilst a 

 lens of low angle gives penetration and, in consequence, a more 

 general definition over the whole field of view. 



The nose-piece is a useful adjunct for the busy worker, enabling 

 him to rapidly change the powers, and is made to carry two, 

 three, or four objectives. It is especially of advantage in photo- 

 micrography. 



The eyepiece, or ocular, is a combination of lenses placed at the 

 upper end of the tube ; it does not magnify the object per se, but 

 amplifies the aerial image formed by the objective. There are 

 several kinds ; that in common use is called the " Huyghenian," 

 and is composed of two plano-convex lenses, having a stop or dia- 

 phragm in the visual focus of the upper lens. They are made of 

 several powers, denoted by letters A, B, C, D, E, and F, and their 

 initial magnifications are respectively 4, 6, 8, 10, 12, and 15 diam. 

 The most useful power for general work is the B eye-piece. The 

 "compensating" eyepieces are designed primarily for use with 

 apochromatic objectives, being specially constructed to correct cer- 

 tain shortcomings in the latter. The " holoscopic " eyepieces, by 

 Watson & Sons, are for use with either achromatic or apochro- 

 matic objectives, and thus obviate the necessity for two series of 

 eyepieces. The " projection " eyepieces, as the name implies, are 

 of the greatest use in high-power photomicrography, and are used to 

 project the image on a screen. 



The substage condenser is for the purpose of condensing light on 

 or through the object examined, and is fitted underneath the stage. 

 It is an essential, part of the modern microscope, but although many 

 of the cheaper instruments are unprovided with them, no microscope 

 should be purchased that has not fittings for such, so that it can be 

 added at any time, for only low power work can be done in its 

 absence. For high power work a good substage condenser is 

 absolutely necessary, as is also a rack and pinion adjustment for 

 accurately focussing on the object the light transmitted. Many 

 eminent microscopists consider the substage condenser second only 



