1734 Journal of Applied Microscopy 



third condenser, and in some cases to a substage condenser. The condensation 

 of the light with its concomitant heat makes it necessary to introduce some form 

 of heat screen between the pair of condensers and the microscope. The special 

 feature of this apparatus is the third condenser, consisting of a plano-convex lens 

 of about the same diameter as the condenser lenses used in the lantern, and of 

 the same or longer focus. A little experience will show the need of such a lens, 

 if theoretical considerations do not satisfy the mind. In adjusting the oxyhydro- 

 gen light for lantern slide work the aim is to secure maximum illumination of a 

 surface three and a half inches or more in diameter with rays of light converging 

 at such an angle that the entire cone of rays will enter the projection lens when 

 at its correct focal distance from the slide. The aperture of a quarter size 

 Darlot projection lens is about 15 g inch. If now an examination of this cone 

 be made, it will be found that there is no sharply defined and very bright princi- 

 pal focus. In microscopical projection we have to do with fields ranging from 

 ^^2^ inch in diameter in a one-inch Wales projection objective to j\ inch in a 

 No. 3 Leitz, about J^ inch in a No. 6 Leitz, and about J^ inch in a B. & L. 

 yV inch oil immersion. All the light which is to fall upon an area on the screen 

 of from three to fifteen feet diameter, depending on the magnification, must pass 

 through these small fields in the objects. The diameter of the front lens in a 

 ^L inch oil immersion is only about ^-^ inch, and yet enough light must pass to 

 give a bright picture covering a diameter of fourteen feet on the screen when a 

 magnification of 9400 diameters is used. These considerations show the abso- 

 lute importance of careful attention to the condensers. A word of caution is 

 needed here, however, because of failures due to the use of condensers which 

 give a pencil of rays having too wide an angle for the objective which receives 

 them. In such cases the outer rays of the cone are not transmitted by the lenses 

 of the objective, but are absorbed by the metal of the objective, with consequent 

 danger from overheating the objective as well as imperfect illumination of the 

 picture. Specific directions for adjusting the light and lenses when oxyhydrogen 

 is used will be given when treating of the electric arc projection microscope. 



The rapid development of electric light and power plants during the last few 

 years has brought the most satisfactory artificial light within easy accuss of many 

 thousands of people who have used, or wish to use, the projection microscope. 

 With the electric arc light, and a suitable system of condensers, it is possible to 

 obtain pictures on the screen with a magnification of from 5000 to 8000 diame- 

 ters. The same rules in reference to condensers and heat screen apply as in 

 using oxyhydrogen. Either direct or alternating currents may be used,- -the 

 objection to the latter being the continuous humming sound from the arc. A 

 rheostat, suited to the voltage of the current, is used with the electric lantern. 



We now come to the consideration of sunlight, the cheapest, as far as neces- 

 essary apparatus is concerned, easiest to use, and by far the most powerful. All 

 forms of projection work which can be done with the various artificial lights may 

 be accomplished in a more satisfactory way with this natural light, and more than 

 double the magnification is possible with high-power objectives. The author 

 regularly uses a B. & L. Jj ^^^^ oil immersion lens which, with an amplifier, 

 gives a magnification of 9400 diameters, and tests of similar lenses giving 10,000 



