Reade, on a New Hemispherical Condenser. 59 



round the orifice of the tube. The slides or discs are then to be 

 heated in the flame of the lamp^ so as to drive oiF the 

 moisture, and raise considerably the temperature, of the 

 glass. The slide or disc, thus heated, is to be placed over the 

 mouth of the tube, and kept there till bright spots appear on 

 its under surface. The spirit-lamp is then to be removed, 

 and the glass allowed to cool. The process may be conducted 

 with the two hands over the lamp, or 

 the holder may be supported on a retort- 

 stand, as is shown in figure 32, and the 

 spirit-lamp removed for a time after each 

 operation. Good results can only be 

 obtained when the slide or disc is heated ; 

 but if too much heat is used, the crys- 

 tals are dissipated as soon as formed. 

 When the operation is quite successful, 

 we obtain one of the most beautiful of 

 microscopic objects, and one of the very 

 best illustrations of the value of the binocular microscope 

 as a means of identification and diagnosis."^ 



On a New Hemispherical Condenser for the Microscope, 

 and its use in illustrating an important principle in 

 Microscopic Illumination. By the Rev. J. B. Reade, 

 F.R.S. 



(Read May 8tli, 1861.) 



The condenser which I am now using has been so favor- 

 ably received by several of my friends, that I am induced, at 

 their request, to offer a description of it to the members 

 of our society. I need scarcely say, that an unpretending 

 single lens cannot be proposed as a rival to the almost per- 

 fect combinations in use among us; but it may, perhaps, 

 take its place as an efl&cient adjunct to the microscopes of 

 those observers who are disinclined, from one consideration 

 or another, to procure more expensive apparatus. 



The condenser consists of a hemisphere of glass, about 

 one and three-quarter inches in diameter, with an arrange- 

 ment of stops by which difficult test objects are well defined 

 under half-inch object-glasses of 90° aperture. It is set 

 in a thin brass ring, and screws upon a cylinder adapted, like 

 other fittings, to the opening of the sub-stage of the microscope. 



* Tor a more detailed description of the mode of obtaining crystals of 

 airsenious acid, consult a paper in 'Beale's Archives/ No. iir, 1858, and the 

 second edition of my 'Principles of Forensic Medicine,' in which several 

 of the forms depicted here will be found figured. 



