Optical Ghosts. 35 



been better written, and confined to its legitimate subject. 

 Mr. Dircks complains of others, and probably with reason ; 

 but about quarrels of this kind the public care little, and when 

 they pay their money for the little book entitled " The Grhost 

 as Produced in the Spectre Drama, by Henry Dircks, Civil 

 Engineer," they do not expect to find nearly all of it devoted 

 to a partially intelligible account of grievances with which 

 they have nothing to do. The amount of explanation given 

 will prove provokingly small, and, to those unacquainted with 

 optics, of little use ; while those who are familiar with that 

 science did not want it at all. Mr. Dircks' merit in the patent 

 ghost business appears to consist in the fact that he saw how 

 to utilize the long-known principles involved in the neutral 

 tint reflector, used by microscopists as a substitute for the 

 more expensive camera lucicla. In this instrument a little 

 plate of thin glass is placed so that the eye looks at it at an 

 angle of 45°, and receives the reflection of the image which 

 the microscope forms of the object on the stage. Thus the eye 

 is affected, not quite so strongly, but just in the same way as 

 if it had looked straight down the microscope tube ; and if a 

 piece of white paper is held below the reflector, the object 

 will appear projected upon it, and the eye can, in addition to 

 receiving the reflection, look through the glass and see the 

 hand and pencil by which the outline is traced. 



To make this kind of action plainer, let a few simple 

 experiments be performed, and let the reader remember that 

 the angle of incidence is always equal to the angle of reflec- 

 tion, and that objects seen in a looking-glass seem just as far 

 behind it as they are actually before it. If any of our young- 

 readers do not distinctly understand the angle of incidence 

 question, they can easily resolve it with marbles or bagatelle 

 balls. Let them place a box, with square sides, on the table, 

 and make a chalk line, so as to form a perpendicular to one of 

 its sides, and falling on the centre. Then, if a marble is 

 bowled against the box so as to strike it slantingly on one 

 side of the perpendicular, it will be thrown back in a similar 

 slant on the other side of the perpendicular. The rays of light 

 behave like the marble or bagatelle ball in this respect. 



For our first experiment, take a hand looking-glass, and 

 see your face in it ; then incline the bottom of the glass away 

 from you till your face is quite, ost, and then your body, or 

 hand, if in the way, will appear plainly. You lose sight of 

 the reflection of your face because the angle of the rays from 

 it which fall upon the glass is such that the resulting angle of 

 reflection sends them away from you. You see your body, or 

 hand, because the angle of their incident rays is such that the 

 resulting angle of reflection carries the image straight to your 



