14 



The upper and lower sets of beaded ribbing cross each other 

 at an acute angle, and their general direction is somewhat 

 wavy. The markings are caused by the cross intersections 

 of the ribbing, and exactly imitate the effect of watered silk 

 caused by pressing tw^o pieces powerfvdly together. 



Minute aplanatic lenses being employed on the stage — 



A. Image of a watch, 8 feet distance. Some objectives 

 show the image enveloped in yellow fog ; the time cannot be 

 distinguished. No focusing or " collar adjustment" gets rid 

 of the nebulosity. 



A small bright aperture appears nebulous and radiating ; 

 either the brightest part of the radiant cone is before or behind 

 the most distinct image. The images of two contiguous bright 

 apertures coalesce and cannot be divided as double stars. 



B. The divisions of a micrometer imaged by the minute 

 lens can scarcely be discerned in a haze of nebulous light : 

 the micrometer was placed on the plane mirror illuminated 

 by a bright cloud. 



0. Rows of beads appear cylindrical bodies unless the 

 aberration is finely corrected. 



D. The image of a church clock, 200 feet distance, can be 

 formed by the plane mirror, and the stage lenses will give the 

 details of the face, the small dots for the minute hand, and, 

 of course, the figures are beautifully displayed, sharply cut 

 and defined or lost in mist according to the size of the stage 

 lenses and the power employed, and the corrections of the 

 objectives. 



Other Tests. 



Podura scales. As I had the honour of communicating 

 to the Royal Microscopical Society in May last an account of 

 the podura beads existing on both sides of the basic membrane, 

 I may be permitted to draw attention to the nearest approach 

 made to the definition of this difficult object as given by Mr. 

 Richard Beck, who has figured the cylindrical bands of the 

 Podura, but described them as out of focus and a false appear- 

 ance, whereas I always find these bands to present their pro- 

 visional appearance as heralding the development of their 

 beading. 



Lepisma saccharina. The exceedingly beautiful striaj 

 of this scale are also shown, in the same work, as perfectly 

 transparent and clear, like threads of glass. These cylindrical 

 ribs are also composed of spherical beads, and they exist on 

 both sides of the scale, the lower set radiating from the quill 

 being much smaller than the upper set. 



