



146 BECORD OF CURRENT RESEARCHES RELATING TO 



and effectually answering its purpose, without any difficulty in use. 

 The idea has long been exploded that it is necessary to use as illu- 

 minators achromatic lenses having but little spherical aberration ; 

 lenticular arrangements uncorrected for chromatism act quite as well — 

 in fact, some of the best effects are by the chromatic bands of ordinary 

 lenses — and for the purpose of intensity mere condensation, irrespec- 

 tive of any image-forming focus, is quite sufficient. 



In March 1856, Mr. Wenham described several forms of immersion 

 illuminators. The first was a right-angled prism connected with the 

 slide by an intermedium of oil, turpentine, or oil of cloves, which, 

 however, had the fault of requiring the addition of some extraneous 

 means of condensing and intensifying the light ; for this reason a 

 nearly hemisj^herical lens was used, connected with the slide, in the 

 same way as the prism. This lens gave more light, and was in every 

 way preferable. A cylinder and a cone of glass were also tried, and 

 found to be a great improvement, but there still remained a difficulty 

 iu concentrating the most light towards the object above the centre 

 of the base. A narrow line of light thrown across the object is very 

 „ „ efficient for the purpose of developing striae, and 

 to obtain this line the form was finally adopted, 

 shown full size in the annexed woodcuts. Fig. 11 

 being a side view, and Fig. 12 an edge view. It 

 consists of a semicircular disk of glass of ^ inch 

 radius ; the edge is rounded and well polished to a transverse radius 

 of y\f inch, for the reason that the focus of a spherical surface on 

 crown glass falls within its substance to nearly three times the radius, 

 consequently the line of light will be in the most concentrated position 

 at ^j^ inch above the centre of the semi-disk, which distance is 

 sufficient to reach objects mounted on slides of the usual thickness, to 

 the under side of which it is connected with water or more refractive 

 oil. The sides of the semi-disk are grasped by a simple kind of 

 open clip attached to the substage. This illuminator is complete in 

 itself, and requires no supplementary condensing lens ; the obliquity 

 is simply obtained by swinging the ordinary mirror sideways, and by 

 this means the Amphipleura mounted in balsam can be at once resolved. 

 The disk illuminator is very easy to make. The polished edges 

 of two suitable pieces of glass are cemented together with shell-lac. 

 One face is then grotnd flat and attached to a brass chuck with 

 sealing-wax, seeing that the line of junction falls in the centre ; the 

 disk is then turned or ground down to a circle h inch in diameter. 

 The edge is next rounded and ground true by means of a piece of 

 brass having a channel cut in it of -j^(jth radius, then smooth with the 

 finest emery, rocking the brass grinder and reversing it frequently 

 to equalize the curve. The polisher is a stick of wood with a yV*^ 

 radius notch cut in the end ; this is held on the T rest, and pressed 

 on the edge of the glass with a rocking motion from side to side, 

 occasionally turning it over, using crocus and water, and as it is 

 important that there should be no rings visible in the polish, the last 

 finish must be given by melting some hard pitch or polishing-wax 

 into the groove in the stick, and finish with this and fine rouge. The 

 half-disks, when separated, produce two illuminators of equal form. 



