Using the Microscope 285 



ARTIFICIAL LIGHT 



During a considerable part of the year daylight is often insufficient 

 for successful work with the microscope. Numerous contrivances 

 for artificial illumination have been devised, some of them fairly good, 

 but most of them thoroughly unsatisfactory. More than two hun- 

 dred years ago Hooke used a device for artificial illumination which 

 probably suggested the apparatus used by the late Professor Stras- 

 burger at Bonn. The apparatus in use in our own laboratory is 

 only a slightly modified form of that used in the Bonn laboratory. 



The apparatus consists, essentially, of a hollow sphere filled with 

 liquid. A fairly good and practical light can be got with an ordinary 

 lamp by allowing the light to pass through a wash bottle filled with a 

 weak solution of ammonia copper sulphate. A piece of dark paper 

 with a circular hole in it serves as a diaphragm, and at the same time 

 protects the eyes from the direct light of the lamp. Such an arrange- 

 ment is shown in Fig. 105. Wash bottles, however, are not perfectly 

 spherical and the mounting is not convenient. To secure a perfectly 

 spherical globe, it was necessary to have a mold made. The globes, 

 as we now use them, are of the finest flint glass, have a diameter of 

 15cm. (6 inches), and are mounted in a convenient black frame, 

 Fig. 106. The globe acts not only as a condenser, but also as a ray 

 filter. For general laboratory work and for nearly all research work, 

 a weak solution of ammonia copper sulphate has proved most satis- 

 factory. The solution (to fill one 15 cm. globe) may be made by 

 adding 50 c.c. of ammonia to 25 c.c. of a 10 per cent solution of copper 

 sulphate, and then adding enough distilled water to fill the globe. 

 If a white precipitate appears and makes the solution look milky, 

 add more ammonia. The strength of the solution depends so much 

 upon the power of the light that no fixed formula can be given. 

 Simply dissolve in water a small crystal of copper sulphate about 

 as large as a grain of corn then add about 50 c.c. of ammonia, and 

 then add distilled water until a light, clear-blue solution is secured. 

 With a very strong light, the solution may have a rather deep-blue 

 color; with a less powerful light, the solution must be weaker. 



In studying the extremely difficult achromatic structures con- 

 cerned in nuclear division, a light violet solution of permanganate 



