MEMORANDA. 233 



in which he says, " the opinions of observers not only differ 

 from one another, but are so utterly and diametrically at 

 variance, that if one side be rig^ht, the other must be altogether 

 wrong." — G. Hunt, Handsioorth, Birmingham. 



Definition of Delicate Test Objects. With Powell and Lea- 



land's l-4th objective, and the A eye-piece, the magnifying 

 power being 200 diameters, using also their improved achro- 

 matic condenser. No. 8 aperture, without the stop for the 

 central rays, I have clearly defined the double set of lines at 

 the same time oi Navicula ciispidata and Pleurosigma Fasciola, 

 also the fine markings of Navicula ohscura and P. delicatula^ 

 and the still more difficult ones of Nitzschia sigmoidea. 



With the 1-8 th objective and the same eye-piece, the mag- 

 nifying power being 400 diameters, I have brought out the 

 dots of Pleurosigma obscura and P. delicatula as plain as in 

 the p. angulata, and also the very delicate markings of the 

 Amician test, Grannnatophora subtilissima, and Navicula 

 rhomhoides and (^Plenro^igma ?) macrum. In fact the Diato- 

 maceae generally are brought out in a most distinct manner. 



The angular aperture of the l-4th objective is 95°, of the 

 l-8th, 125°, and of the condenser, 105°. 



The microscope is placed at the polarizing angle 56° 45'. 

 The illumination is that of the small camphine lamp, the base 

 of the flame being 7| inches from the level of the table, the 

 flat side of the mirror being uppermost, and nearly horizontal, 

 and 2^ inches below the base of the flame, and at a distance 

 from it of about If inches. Tlie achromatic condenser is 

 racked up very near to the object: with this preliminary 

 arrangement, by a slight movement of the mirror and con- 

 denser, a small triangle of brilliant light is obtained at the 

 base of the field ; above the bright light is a deep yellow, 

 filling the space of more than half the remainder of the field, 

 tlie remaining portion at the top being very dark ; on the yellow 

 portion of the field the object is so arranged, that the intense 

 light from the bright angle may be thrown upon it, and then 

 the delicate markings immediately appear. This diagram 

 may explain it more clearly. 



A, the angle of bright light. B, yellow 

 ground. C, dark ground. 



The objects are placed in either position 

 as placed in the diagram. 



The markings of {Pleurosigma T) macrum, 

 N. rhomhoides, the Amician test, and Gram- 

 matophora, are made still more distinct by 

 placing stop 1 as marked in the portion A of the diagram. 

 — E. L. 



