496 SUMMARY OF CURRENT RESEARCHES RELATING TO 



free and not be troubled to keep one eye closed — it is much better, in 

 fact, to keep both eyes open. This is done in using either of the in- 

 struments ; in the smaller one the shaped sliding head-rest keeps all 

 light from reaching the eyes except through the single revolving dia- 

 phragm ; in the larger one there are two revolving diaphragms, one 

 (shown at D) for each of the eye-pieces E E. The eye that is not under 

 observation is kept in complete darkness by turning D until the index 

 marks zero ; at this position there is no aperture in the diaphragm. 

 Thus either eye can be occluded with ease. The mirror M is plane 

 and not concave, and made sufficiently large to cover the whole of the 

 ground-glass G with a flood of light reflected from a neighbouring 

 incandescent gas lamp or other source of light. As the mirror is 

 carried by the stage and moves with it, the illumination of the field 

 remains unaltered in adjusting the inclination of the pillar. The pillar, 

 stage, and mirror move with stiff friction round the centre A, and 

 can be clamped in any position. In order to avoid any shifting of the 

 observer's head, and also to avoid fatigue, a hinged and padded head-rest 

 H is fixed in such a position that the forehead rests comfortably upon it. 

 The head-rest is also made to rise and fall, and there is an arrangement 

 for accommodating the diaphragms to the distance between the eyes.* 



Wilsing, J. — Uber die Bildebenung bei Spektographen-Objektiven. 



[Shows how H. Hartmaun's equations and conditions can be almost exactly 

 satisfied by choice of certain kinds of glass.] 



Zeit. f. Instr., xxvi. (1906) pp. 101-7. 



(6) Miscellaneous. 



Construction and Fittings of a Microscope Room. t — The following 

 extracts are taken from the report of N. A. Cobb, who describes very 

 fully the construction and fittings of his Microscope room at Honolulu. 

 The report also deals with the illustration room, dark room, and the 

 camera-lucida. 



THE MICROSCOPE ROOM. 



For many years it has been customary in the best laboratories to 

 mount various instruments of precision upon pillars of stone or masonry 

 deeply imbedded in wells in the ground and passing upward through 

 the floors of the laboratory without contact. The object of this arrange- 

 ment is to prevent tremors. It is not often that the Microscope has 

 received such special attention, but wherever high powers are used and 

 especially when photo-micrographs are being prepared, or whenever 

 high-power camera-lucida drawings are being made, the reduction of 

 vibration is an important factor in the success of the work. For many 

 years the writer has had Microscopes mounted in this way, and hereby 

 testifies strongly in favour of this method of using the Microscope. 



The plan is carried out in cement and steel (see fig. GO). Below the 



* For the loan of the blocks in this Plate we are indebted to the courtesy of 

 the Royal Dublin Society. 



t Rep. Exper. Stat. Com. Hawaiau Sugar Planters' Assoc, 1905, pp. 39-59. 



