208 PROCEEDINGS OF THE SOCIETY. 



(3) Body txibe should be short enough to allow of an effective tube 

 length of 1(50 mm. (Gg in.), including the space occupied by the illu- 

 minator. A draw tube allowing the total tube length to attain 9 in. 

 should be provided. No focussing movement for the tube is required. 



(4) Illuminator. — The " vertical illuminator " should be incorpor- 

 ated in, or permanently attached to, the lower end of the body tube. 

 The illuminator may either be made to rotate or be so arranged that 

 light may be admitted from either side or from the front. The reflector 

 should be mounted on a substantial fitting provided with a large milled 

 head, and the reflector should be removable from the instrument, for 

 purposes of interchanging different kinds of reflectors, without disturb- 

 ing the objective. Totally-reflecting prism, clear glass disc and silvered 

 glass half -disc reflectors should be provided. The illuminator and the 

 lower end of the tube should be so arranged that the reflector can be 

 brought down, or is normally placed, immediately above the back lens 

 of the objective. The reflector should be adjustable in the direction at 

 right angles to the optic axis, and should be set by an adjusting screw. 

 The illuminator should be provided with an iris diaphragm, the position 

 of which can be varied. 



(5) An Additional Illuminator is required for use with very low 

 powers ; this should be supplied in the form of an arm attachable to 

 the body tube or limb, and carrying a holder for either (or both) an 

 opaque silvered reflector or (and) a thin plate of parallel glass — both 

 these reflectors to be used between the objective and the specimen. 

 Adjustment must be provided to allow this outside reflector to be 

 brought down near the surface of a specimen at the extreme bottom 

 position of the stage. 



(6) Objectives. — Very low power ... 75 to 100 mm. 



(Low Power ... 50 mm.) 



Medium ... ... 16 mm. 



(do. ... ... 8 mm.) 



High ... ... 4 or 6 mm. 



(Oil Immersion ... 2 mm. or 3 mm.) 



(The objectives in brackets to be regarded as extras — not 

 required by students.) 



The objectives to be mounted in very short mounts (upper end as 

 nearly as possible flush with the back surface of the top lens), to be 

 corrected for use without cover glasses, and selected of a type avoiding 

 internal reflections. 



(7) Ttvo E'//e-Fieces, to be selected with magnifications of X 6 or 8 

 and X 10 or 12. If of the compensating type, to be selected with 

 magnifications X8 and 12 or 18. 



