ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 621 



making the preparation as thin as possible than in the introduction of 

 what is likely, in any but the most skilful and experienced hands, to 

 prove a new source of error. Every one who has examined muscular 

 fibre, for example, under a dry objective of very high power and large 

 aperture, well knows that so great an alteration is produced in its 

 aspect by the slightest change in either the focal adjustment or the 

 cover-correction that it is impossible to say with certainty what are 

 the appearances which give the most correct optical expression of its 

 structure. This being a matter of judgment on the part of each 

 observer, it seems obvious that the nearest approach to a correct view 

 will be probably given by the focal adjustment of the best homogeneous 

 immersion-objectives, in fixed settings, to the plane of the preparation 

 immediately beneath the cover-glass." 



Lighton's Immersion Illuminator.* — This device of W. Lighten 

 (fig, 101) consists of a small disk of silvered plate-glass c, about 1/8 in. 

 thick, which is cemented by glycerin or some homogeneous- immersion 

 medium to the under surface of the glass slide s, r being the silvered 

 surface of the disk, b the immer- 

 sion objective, and /the thin glass Fig. 101. 

 cover. The ray h from the 

 mirror or condenser above the 

 stage will enter the slide, and 

 thence be refracted to the sil- 

 vered surface of the illuminator r, 

 whence it is reflected at a corre- 

 sponding angle to the object in 



the focus of the objective. A shield to prevent unnecessary light from 

 entering the objective can be made of any material at hand by taking 

 a strip 1 in. long and 3/4 in. wide, and turning up one end. A hole 

 of not more than 3/16 in. in diameter should be made at the angle. 

 The shield should be placed on the upper surface of the slide so that 

 the hole will cover the point where the light from the mirror enters 

 the glass. " With this illuminator Moller's balsam test-plate is 

 resolved with ease, with suitable objectives. Diatoms mounted dry 

 are shown in a manner far surpassing that by the usual arrangement 

 of mirror, particularly with large angle dry objectives." 



Illumination by Daylight and Artificial Light— Paraboloids and 

 Lieberkiihns.t — E. M. Nelson finds daylight effective for low powers 

 up to 2/3 in., and with condenser up to 1/6 in. Direct sunlight 

 involves the use of a heliostat, otherwise the continued adjustment 

 of the mirror is irksome. Where strong resolving power is needed, 

 oblique pencils of sunlight from the heliostat outrival any other 

 illumination ; but much care is necessary not to injure the sight, and 

 on the whole, he cannot recommend its general use except for photo- 

 graphing. Diffused daylight is too uncertain and too variable for 

 accurate testing of objectives. It is not possible to get with diffused 

 daylight the absolutely best image that an objective will produce, 



• Amer. Mon. Micr. Journ., v. (1884) pp. 102-3 (1 fig.), 

 t Engl. Mtcli., .\xxix. (1884) p. 48. 



