162 MICEOSCOPIC VISION. 



unites as well portions of the 1st order with the dioptric 

 beam D (Fig. 4), thus rendering their separation impos- 

 sible.^ 



There is another form in which the small cone is met 

 with, viz. " oblique illumination." 



This kind of illumination is very old, and was probably 

 invented by Dr. Groring,^ who in 1826 found that by means 

 of oblique light lined tests could be resolved more easily. 

 By oblique illumination is meant illumination by beams in 

 one or two azimuths, inclined more or less to the axis of the 

 microscope. Often the inclination of the illuminating beam 

 is so great, that it only just falls within the grip of the 

 objective ; if its obliquity were a little greater, the object 

 would appear illuminated on a dark ground. With oblique 

 illumination the dioptric beam passes through a marginal 

 zone on one side, and when the structure S^ S, 48,000 per 



^ In Fig. 4 the dioptric beam and the spectra of the 1st and 2nd 

 orders are distinguished by being drawn in steps, those portions 

 which overlap being indicated by thick lines. The structure 0, 0, is 

 supposed to have 12,000 lines or marks per inch, and to be similar to 

 that in Fig. 2. P, P, in Figs. 3 and 6, are supposed to have 24,000 

 lines per inch, while S, S, in Fig. 5, has 48,000. In thes3 figures the 

 refraction of the lens has not been represented. The lenses have 

 merely been inserted to show the position of the spectra with regard 

 to them. 



^ " On Mr. T alley's thick aplantic object glasses for diverging rays: 

 with an account of a few Microscopic Test objects by C. B. Goring, 

 M.D." Quarterly Journal of Science, Literature and the Arts, Dec, 

 1826. In vol. xxii. of The Journal of the Royal Institution, published 

 1827. 



Note. — Mr. Tulley (also spelt TuUy), the celebrated telescope 

 maker, was the first to construct (in 1824) a successful achromatic 

 microscope object glass in this country. It was a single uncemented 

 triple combination composed of a biconcave flint enclosed between 

 two biconvex lenses, one being of crown, and the other of plate glass. 

 It was -O-inch focus, and -16 N.A. in aperture. 



