.274 Progress in Science. [April, 



(Fig. 18), in a nearly similar manner to Mr. Wenham's combination of para- 

 boloid and truncated lens,* but of course with less loss of light. The following 

 constructive details are given. If the focus be chosen ^th of an inch above the 

 upper surface, the equation of a perpendicular section y* = ax will become 



y* = a (-+ — J; and solving the equation for a, we shall have 



a= —\+ v' 4y 2 + A 5 and if the upper surface of the paraboloid be made fths 

 of an inch in diameter (a size most convenient in practice), a becomes = to £, 

 and the equation j 2 = §* is that of the paraboloid form. Should the focus be 

 taken at a distance of ^th of an inch above the truncated paraboloid, and its 

 upper surface be i inch in diameter, then the equation becomes yi = %x quam 

 proxime : this latter form will admit of a hollow cone of light of 120 to 185 , 

 and will almost give a dark ground illumination for a ^th immersion object 

 glass. A modification of the same principle is also given by which objects 

 may be illuminated from above and the use of the highest powers permitted. 

 Dr. Barker further considers that the immersion principle is applicable to all 

 kinds* of condensers placed in the axis of the microscope. 



The thinnest kind of glass used for covering microscopical objects is very 

 troublesome to clean on account of its brittleness ; when slight friction will 

 remove the dirt it is best rubbed between two discs of wood or metal covered 

 with wash leather, but if cleaning with alcohol or other fluid is needed, accom- 

 panied with somewhat hard rubbing, the breakage is considerable. The fluid 

 used by photographers for cleaning glass plates, composed of sulphuric 

 acid 1 oz., bichromate of potassium 1 oz., water 1 pint, will be found very 

 effective. The thin glass is to be left in this solution for some hours, — a day or 

 two will do no harm ; it can then be poured off and the glass washed with 

 several waters to remove all traces of the preparation, the last washing being 

 with distilled water. The thin glass is placed upon a soft porous clothf spread 

 upon a table and carefully rubbed dry with another cloth ; it will be found that 

 very little breakage takes place, as the glass is so perfectly cleaned even when 

 extremely dirty or covered with bloom, that very little pressure or hard rubbing 

 will be needed. This process may be used with advantage for cleaning slides 

 as well as cover glass ; the solution is cheap, and effectually cleanses all kinds 

 of glass with the minimum amount of trouble. 



Dr. Ward, at the American Association for the Advancement of Science, 

 remarked " that the production of a beaded appearance on the Podura scale as 

 a purely optical effect should be considered no longer doubtful, but rather as 

 an occasional accident to persons using high powers." As an extreme instance 

 in the case of a coarse and familiar structure, he related that, while experi- 

 menting upon an elater of Marchantia polytnorpJia, that beautiful double spiral 

 was resolved into three rows of "beads" or "hemispheres," perfectly distinct 

 and unmistakable, which occupied of course the position of the middle and 

 edges of the spiral. They were illuminated by parallel light, very oblique, 

 under a T Vth objective of 175 , worked at a power of 3000 diameters. 



According to the "American Naturalist," October, 1870, Colonel Dr. J. J. 

 Woodward has succeeded in photographing the beaded appearances in the 

 scale of Degeeria domestica, from specimens supplied by Mr. S. J. Mclntire. 

 They are plainly seen in a series recently received by the Royal Microscopical 

 Society : also the lines of Amphifileiira pellucida [Navicula acus), and the 

 hemispheres of Surirella gemma. Mr. Wenham considers the beads as 

 " ghost beads," and evidently caused by intercostal corrugations of membrane. 



Mr. T. H. Hennah, of Brighton, has succeeded in procuring a series of 

 photographs of arrangements of intersecting glass rods, as suggested by Dr. 

 Pigott : the results are curious in the extreme ; they were placed before the 

 writer without his being informed of their nature, and were at once pronounced 

 to be diatom hemispheres, insect eyes with diminutive images of objects, and 



* Monthly Microscopical Journal, 1869, vol. ii., p. 28. 



+ Old cotton stocking, carefully cleaned with soda and hot water to remove the soap from 

 former washings, answers well for this purpose. 



