2()6 Transactions of the Society. 



XV. — The Vertical Illuminator and Homogeneous Immersion 

 Objectives. By J. W. Stephenson, F.E.A.S., Treas. E.M.S. 



(Head 9th April, 1879.) 



The Fellows will have seen in the April number of the Journal 

 (p. 194) a note extracted from the 'American Naturalist' for 

 February, in which are described the advantages found by Mr. 

 Morehouse, of New York, to be obtained from the use of the Vertical 

 Illuminator * in the resolution of Diatoms and Podura scales. 



On reading the note, I tried the apparatus on both classes of 

 objects, and can fully endorse the statement made as to the sur- 

 prising results obtained. Slides of A. ])ellucida which were 

 deemed worthless because all the striae had, as was supposed, been 

 destroyed in cleaning, were resolved with the greatest ease, and 

 Podura showed parallel light or white lines from one end of the scale 

 to the other, somewhat reminding one of Lepisma. The Vertical 

 Illuminator was soon after its first invention discarded by practical 

 microscopists on account of the amount of fog which was caused 

 by the reflection, at the upper surface of the cover-glass, of the 

 rays transmitted through the objective. It is obvious that this fog 

 will not be observed when an oil-immersion objective is used, as in 

 that case the front lens of the objective, the intervening stratum 

 of oil, and the cover-glass itself, are all optically continuous, so 

 that the upper surface of the cover-glass has optically ceased to 

 exist, the only reflection being from its under surface when dry 

 objects are used. An additional advantage is therefore found for 

 homogeneous-immersion objectives. 



My object is not, however, to deal with this branch of the 

 subject, but with an entirely diflerent application of the Illuminator, 

 not noticed by Mr. Morehouse, but which appears to me to be of 

 great scientific interest. 



This point is the visible demonstration which the Vertical 

 Illuminator affords, not only that many modern objectives, and 

 notably those on the homogeneous-immersion system, have angles 

 far exceeding the equivalent angle of 180°, but also that the 

 extent to which this excess is in any particular case carried, can 

 at once be appreciated. 



The existence of this excess, although at one time doubted, has 



* As several inquiries have been made as to what instrument is meant by the 

 " Vertical Illuminator," I may refer to Dr. Carpenter ' On the Microscope,' 5th erl., 

 p. 153, where the instrument is both described and figured. A small silver 

 speculum (Professor Smith), or a movable disk of thin glass (Messrs. Beck), or a 

 piece of parallel glass fixed at an angle of 45° (Messrs. Powell and Lealand), is 

 fixed in a short tube (with a side aperture) interposed between the objective 

 and tlie body of the INIicroscope, by which means a pencil of light entering at the 

 aperture and striking against the spccuhnn or inclined surface of the disk or plate, 

 is reflected downwards through the objective ujjon the object. 



