MICRO-OBJECTIVES AND COMPENSATING OCULARS. 159 



microscope. A great saving of time, labour, and 

 annoyance can thus be effected by these low-power 

 eyepieces, especially when using immersion lenses. 



(2) The ordinary working oculars, magnifying 

 respectively 4, 8, 12, 18, and 27 diam. ; their focal 

 lengths vary from 45 to 10 mm. for the Continental 

 stand, and from 67 to 10 mm. on the English lOin. 

 tube. 



(3) Oculars for projections, magnifying 2 and 4 

 diam. for a tube of 160 mm. and 3 and 6 diam. for 

 a tube of 250 mm. or 10 in. long respectively. These 

 oculars have two diaphragms each, to reduce the 

 effective apertures of the high-power lenses should 

 such be desirable. 



They are constructed for photo - micrography and 

 for the lantern microscope, and yield an evenly 

 illuminated flat field and a well - defined image 

 at any screen distance. They can also be used 

 advantageously with the ordinary achromatic micro- 

 objectives. 



The mountings of these oculars are so arranged 

 that the lower foci of all those belonging to one 

 series are lying in the same plane, so that when 

 interchanging them no new focussing of the objec- 

 tive is required, the optical tube length remaining 

 the same. 



I subjoin tables of Dr. Carl Zeiss' new apochro- 

 matic objectives showing their numerical apertures, 

 equivalent focal lengths in millimetres, and their 

 magnifications at 250 mm. It will be seen from 

 them that the whole series for both Continental 

 and English tubes together consists of only 11 

 lenses, and that 3 dry, 1 water immersion, and 1 

 (or 2) homogeneous immersion lens, in all 5 apochro- 

 matic objectives, would constitute a complete series, 

 and in conjunction with the new compensating eye- 

 pieces would do all the work for which hitherto 

 often from one to two dozen objectives were 

 required. 



The homogeneous immersion lenses are constructed 



