80 



Modern Microscopy 



tube-length can be used on the 10-inch tube, and those 

 for the 10-inch at the 6-inch tube-length, without detri- 

 mental effect ; but in the former case about half must be 

 added to the product of the multiplication of the power 

 of the objective and eyepiece, and in the latter case about 

 one-third must be deducted, in order to arrive at the 

 magnifying power (see ante, p. 69). 



Holoscopic and Universal Eyepieces. 



These eyepieces are made by J. Swift and Son and 

 Watson and Sons, and are intended to be used with objec- 

 tives of both the apochromatic and achromatic types. The 

 lenses used are made of a selected optical glass, which 

 produces a degree of over-correction similar to that asso- 

 ciated with compensating eyepieces 

 when the separation between the eye 

 and field lenses is increased. In 

 order that this may be conveniently 

 effected the eye lens is attached to an 

 inner, or draw-tube, sliding inside the 

 outer tube which fits the microscope 

 body. When the eyepiece is closed 

 together it becomes of the Huy- 

 ghenian type ; when the eye lens 

 tube is pulled out, it gives the effect 

 of a compensating eyepiece. The 

 amount of over - correction can be 

 exactly obtained by the greater or 

 lesser extension of its draw-tube, a 

 scale being provided to record results. This eyepiece is, 

 therefore, applicable to all classes of objectives, and, being 

 made in a useful range of magnifications, will be found 

 a desirable pattern to start with, and acquaintance with its 

 working will lead to greater appreciation of it. It yields 

 excellent effects photographically. Even if it be intended 

 to limit the equipment to achromatic objectives, this type 

 of eyepiece will generally be found to present points of 

 superiority over the ordinary Huyghenian pattern. 



FIG. 21. HOLOSCOPIC 

 EYEPIECE. 



