8th Febry 1826
I have found the notes I made at Munich concerning Fraunhofer’s <1> method of making object glasses.
They are not very important, but I send them to you such as they are.
To grind the glass it is fixed at the extremity of a radius of the exact required length–– This Radius is covered with paper to protect it from the warmth of the hand. The center of motion is a steel spherule A moving in a cup B–– This cup is fastened to a massy stone pillar. [illustration] It is lowered by a screw till the convex glass which you are grinding presses upon a concave one – The two are ground together till they act no longer, when it is necessary to screw the cup a little lower down, & so on.
If the glass were fastened to the metallic lower end of the Radius, the different expansibility of metal & glass would force the latter into a strained position. It is therefore placed upon another glass, & between them are put a number of little lumps of rosin or beeswax; they are the placed in a sandbath, the wax softens, & the glass sinks by its gravity into an unconstrained position.
The two glasses are similarly placed upon a third and the latter fastened to the metal.
The largest & smallest lenses are not ground in this way. The Glasses are polished with Red Oxide of Iron of particular preparation, this is the only process from the beginning of the polishing to its completion. The Oxide is fused upon a scored cake of rosin pressed on the glass by a lever that moves it slowly backwards & forwards. It presses by no means on the center of the glass, but very excentrically. The glass has a movement of rotation which communicates itself to the polisher but not with the same velocity owing to their different sizes, by this means the polish is rendered uniformly good in all parts of the Object glass–– The form of the glass may be considerably altered at pleasure during the polishing, by altering the alternate motion of the lever. An object glass of 9 inches diameter takes 4 days polishing. In order the Center the Object glass, Fraunhofer places upon it a little telescope supported by 3 steel points two of which touch the glass in its circumference and the third in the Center, with this he looks at the eyeglass thro’ the object glass, and adjusts it with the help of two screws, he then turns it half round, so that the steel points rest upon the opposite part of the circumference, and observes again, and if it still points to the Center of the eyeglass he concludes that the objectglass is properly centered.
I remain Dear Sir yours most truly
1. Joseph von Fraunhofer (1787–1826), optician, Munich.