GLASGOW SOCIETY OF FIELD NATURALISTS. 137 



diflScult lined tests, provided the stage be very thin to allow of very- 

 oblique illumination. This arrangement is also cheaper than an 

 expensive condenser, but two lamps are required for it. 



Modifications of the rectangular prism are the Nachet and the 

 Amid Prism (Fig. 14), which serve both to reflect and to condense 

 the light. I can highly recommend the Amici prism as an excel- 

 lent piece of apparatus for the resolution of lined objects, as it 

 gives rays of great obliquity if thrown out of the axis, or intense 

 direct light if placed in the axis, of the microscope. This prism, 

 although small, gives sufficient light for magnifications of thousands 

 of diameters, and is very easily used, but it is not achromatic. 



The best appliances for obtaining various quantities and qualities 

 of light for the illumination of transparent objects are the achro- 

 matic condensers. Any achromatic object-glass of medium power 

 placed under the stage with its anterior lens nearest the object, 

 will act as a good condenser. Opticians supply special achromatic 

 combinations for this purpose, having close to the lower lens one 

 or two diaphragms with various apertures and stops to regulate 

 both the quantity and quality of light. The stops are generally 

 six in number, namely, three central dark ground spots of various 

 sizes for excluding the central rays, one lateral stop, one having 

 two slots forming a right angle, and one stop having two slots 

 forming an obtuse angle. In Ross' condenser the pencil of light 

 may be reduced from 110° to 20^ by the circular holes in the 

 upper diaphragm. It is considered the most useful condenser, and 

 suffices for the resolution of the most difficult tests. Powell and 

 Lealand's condenser is similar, but has 170° angular aperture, 

 and consequently a shorter working distance, but it is the best for 

 the highest powers. Gillet's condenser has the diaphi'agm in the 

 shape of a flat ring which stands nearly perpendicular, and 

 whose various apertures can be brought successively under the 

 field lens. If parallel markings are to be resolved, it is evident 

 that the unilateral stop has to be used, and that the condenser 

 has to be rotated until the light falls perpendicularly against the 

 markings or ribs ; for cross markings the right or obtuse angle 

 stops have to be used, for obtaining dark ground illumination 

 under low powers or for great obliquity Of light under high powers 

 the central dark ground spots have to be employed. Mr. Heade's 

 hemispherical condenser consists of two plano-convex lenses with 

 the plane surfaces directed upwards, and yields very oblique and 



