ZOOLOGY AND BOTANY, MICEOSCOPY, ETC. 323 



sists of two small vertical stems, which are set near one another in the 

 lower face of this mount, and work in the interior of the ocular. They 

 penetrate the rim of the micrometer diaphragm without being attached 

 to it. The diaphragm is moved up and down by the action of an endless 

 screw which penetrates it, and whose upper extremity terminates in a 

 large milled head. The periphery of the milled head slightly exceeds 

 the periphery of the lens-mount, and so renders possible easy turning of 

 the screw. A millimetre scale, placed inside the ocular, near one of 

 the stems, indicates the height of the focus, and the milled head is 

 divided so as to give fractions of a millimetre. 



Fig. 44 shows a simplified form, in which the diaphragm is sup- 

 ported only by the screw, which is flattened on one side, and carries a 

 millimetre graduation. The inconvenience is that if, in focussing, the 

 diaphragm is not properly set, the upper lens must be unscrewed, and 

 the diaphragm raised or lowered. These operations must be repeated 

 until success. But, when once found, the focus is not lost. 



The inventor has also tried an intermediate form, which seems the 

 best of all. As in fig. 43, it has an exterior milled head, but, as in 

 fig. 44, it rejects the guide-stems and trusts to the endless screw alone. 



Movable Ocular Diaphragms.* — M. Malassez has designed a series 

 of diaphragms applicable to any ocular, and capable of transforming it 

 into any variety of eye-piece, micrometric or otherwise. He describes 

 several of his attempts to attain his object, and the following are the 

 final results : — 



(i.) Indicator Diaphragm.^ — A piece of blackened cork was introduced 

 into an ocular and placed just above the ordinary diaphragm. The 

 indicator was put on its upper surface, and consisted of a sort of watch- 

 hand pointed at one extremity and pierced at the other ; an ordinary 

 pin passed through the hole secured the indicator to the cork, but the 

 relative largeness of the hole easily permitted movements of the pin in 

 the plane of the diaphragm. To confine these movements within suitable 

 limits, two pin-heads were set in the cork in proper positions. When 

 the indicator was in contact with one of these stops the point was in the 

 centre of the diaphragm opening ; when in contact with the other it was 

 out of the field altogether. By inclining the ocular the indicator could 

 be got to move and assume the desired position. Any slight inequality 

 on the surface of the cork was useful in preserving the indicator in 

 position (fig. 45). In a more perfect form M. Malassez makes every- 

 thing of metal. The internal diameter is small enough to allow in- 

 sertion into any current eye-piece, e.g. those of Zeiss. It is fitted with 

 three or four small springs which, starting from the periphery and 

 bending outwards, serve to keep the instrument steady, while the curved 

 upper extremities enable the finger to withdraw it easily. The indi- 

 cator consists of a small basal piece into a groove of which a fine hair 

 fits. The adjustment of the indicator is as before, and of course it 

 would be necessary to remove the ocular from the draw-tube in order 

 to set the indicator. 



(ii.) Thread Diaphragm. — Of this there are two models. The first 



* Arch. Anat. Micr., 1900, pp. 436-56 (6 figs.). 



t Thread diaphragms date from 1838, and indicator diaphragms from 1848. 

 Vide Quekett, p. 130 (1st edition).— En. 



