210 A. SCHULZE ON A METHOD OF RESOLVING 



a so-called vertical or ojmque illuminator between the nosepiece and 

 the objective. 



The vertical, or opaque illuminator, was originally constructed for 

 viewing uncovered opaque objects with dry front objectives, and it 

 consists simply of a short tube, having a small circular aperture in 

 its side, and a reflector moveable on a horizontal axis inside this 

 tube. On its simplest form, in the patent opaque illuminator of 

 Messrs. R. and J. Beck, the reflector consists simply of a thin circle 

 of covering glass. Messrs. Powell and Lealand use a plate of 

 parallel glass, and Professor Hamilton L. Smith, of Hobart College, 

 U.S., the inventor of this apparatus, uses a small silver speculum, 

 placed on one side of its horizontal axis, whilst in another form 

 there is a small rectangular prism covering half of the aperture. 

 Messrs. Powell and Lealand add a small diaphragm with four circular 

 apertures of various sizes, of which the smallest appears to me to 

 be the most useful, as the larger ones produce too much glare. 



The modus operandi is the following : Place the tube of the 

 microscope perpendicularly, or nearly so, screw the vertical illu- 

 minator between the nosepiece and the immersion objective, so that 

 the aperture through which the light is to fall stands in front. Then 

 place the lamp, the narrow side of the wick turned towards the 

 microscope, right in front of the latter, at a distance of from six to 

 ten inches. After having roughly focussed the lens on the side, 

 adjust the lamp in a vertical direction, so that a line perpendicular 

 to the optical axis of the microscope, drawn through the centre of 

 the aperture of the vertical illuminator, passes through the lowest 

 point of the flame, or just over the top of the wick. Adjust now the 

 reflecting surface of the vertical illuminator on its horizontal axis, so 

 that a distinct image of the flame appears in the field of vision. 

 This image will, of course, appear brightest, and the definition best, 

 when the narrowest side of the flame is turned towards the instru- 

 ment, which can be easily ascertained by turning the oil vessel of the 

 lamp a little round its axis, whilst looking into the microscope. The 

 field is now quite dark, and nothing is seen but a streak of light 

 about a quarter of an inch in breadth, which passes through the 

 middle of it antroposteriorly. If all this has been carefully attended 

 to, and if a diatom, adhering closely to the cover, is moved in this 

 image of the flame, its markings will appear most beautifully and 

 distinctly resolved, provided that they are lying across the path of 

 light. Various little advantages may be gained by a more careful 



