562 SUMMARY OF CUERENT RESEARCHES RELATING TO 



must therefore be given to the axis of reflection, as that the micro- 

 scopic image shall be brought completely outside the foot of the 

 Microscope. 



But the axis of reflection not being vertical — not being conse- 

 quently perpendicular to the surface of the table, the microscopic 

 image is formed on a plane which is obliquely inclined to the optic 

 axis ; Avhilst the object itself being on the stage of the Microscope, is 

 on a plane perpendicular to the object axis. The result of this is 

 that the relative distances which exist between the eye and the various 

 points of the drawing are different to those which exist between the 

 eye and the corresponding points of the object. 



Consider, for instance, two points of the object equally distant 

 from the axis. These two points will necessarily be at equal dis- 

 tances from the eye, whilst in the drawing the corresponding points 

 will be at unequal distances, that on the right being further from the 

 eye than that on the left. These differences will be produced so long 

 as the plane of the drawing is not, like that of the object, perpen- 

 dicular to the optic axis. 



By comparing the position of the table with that of a plane which 

 is perpendicular to the axis, and meeting the table near the foot of 

 the Microscope, we can exactly determine the differences which the 

 relative distances introduce. The table is the further from the plane 

 according as ifc is further from the Microscope, and the result of this 

 is as if the different parts of the drawing had been made on planes 

 perpendicular to the axis, but more and more distant from the eye. 



But as the amplifications of the drawings obtained by the camera 

 lucida are larger according as they are received at greater distances 

 from the eye, it follows that in a drawing made on the table the 

 amplification would increase without limit in proportion as the 

 drawing is distant from the foot of the Microscope. 



This explains the results of the experiments above mentioned. If 

 with the cameras lucidfe of Milne-Edwards and Nachet the divisions 

 of a micrometer are the wider apart, according as they are drawn in 

 positions further from the Microscope ; if a square gives a trapezium, 

 a circle an ovoid, the large base of the trapezium and the large 

 extremity of the ovoid being on the right, it is because from the con- 

 struction of these camerae lucidae the amplification of the drawing 

 increases from left to right in proportion as it is further off from the 

 foot of the Microscope. 



3. Means of Correcting these Distortions. — I assume that it is not 

 desired to change the construction of the camerfe lucidae. It is plain 

 from what has been said, that to obtain a drawing like the object it is 

 necessary that the microscopic image should be collected not on a 

 plane oblique to the optic axis, but on one perpendicular to it. 



It is easy to fulfil this condition. Two methods maybe employed 

 — (1) Either incline the Microscope so that the axis of reflection 

 becomes vertical, or (2) leaving the Microscope in its ordinary posi- 

 tion, draw not on the table but on an inclined plane, the inclination 

 being such that this plane may be perpendicular to the axis of 

 reflection. 



