DRY DIATOMACEOUS TESTS. 211 



regulation of the relative heights of the light to that of the aperture 

 of the illuminator, or by shutting off one half of the aperture of the 

 latter, or by allowing the light to fall only on the lower half of the 

 aperture, etc., all of which have for their object to let the light fall on 

 only one half of the reflecting surface, leaving the other for the 

 passage of the rays from the object to the eyepiece. The image of 

 the flame does not always intersect the whole of the field, and in this 

 case it falls more in the forepart of the same. The Dallinger lamp 

 gives a high flame, and consequently a very long intense image 

 dividing the field into two halves, whilst the motions of this lamp 

 ensure a speedy and accurate adjustment. Carl Zeiss's water 

 immersion lenses, especially the l-8th ; Powell and Lealand's fine 

 new formula water-immersion, l-8th, and those magnificent oil- 

 immersion lenses*, the l-8th, l-12th, and the 1-1 8th of the former 

 maker, are those I have tried in this way, and with all of these 1 

 have succeeded in obtaining resolutions of the finest-lined diatoms in 

 a way not to be obtained by achromatic condensers, prisms, lenses, 

 &c. ; by transmitted lamplight. 



Amphipleura pellucida resolved in this manner shows its outlines 

 with the greatest distinctness and without appearing in the least 

 degree distorted. The ribs are clearly defined, and the mid-rib 

 reveals a fine black line running along its middle, whilst the cross- 

 strise appear black and beautifully separated, extending over the 

 whole floor of the valve on both sides of the mid-rib, and between the 

 marginal rib, and between the latter and the broad ends of the 

 former. This diatom assumes an olive or light green hue, according 

 to the objective used. Under a power of 400 diams. the markings 

 on the smallest specimen become distinctly visible, and with 

 sufficient amplification they may easily be counted by the flame of 

 an ordinary paraffin lamp, having a wick one half-inch broad. 

 Beautifully as these cross-lines are resolved, I cannot see any longi- 

 tudinal lines, not even with the 1-1 8th oil-lens. I am of opinion 

 that many who honestly believe that they can well resolve Amphi- 

 pleura pellucida will readily grant, after having seen the resolution of 

 this diatom with the vertical illuminator, that they have been labour- 

 ing under a delusion, and that they have only well succeeded on 

 abnormally marked valves. 



Frustulia saxonica, Navicula crassinervia, or Naricitla rhomboides, 



* On Stephenson's system. 



