Fractions are represented horizontally using the slash / symbol. The original separated the numerator and denominator horizontally, separated by a _ line.

Punctuation has been normalized. Except in the following cases, variations in hyphenation, spelling and spacing have been retained as they were in the original publication.

Chapter 2:

Lumer-Brodhum photometer —> Lummer-Brodhun photometer

Chapter 4:

What appear to be hand-drawn primes in the text have been confirmed as: The arc ha´ce —> The arc ha´c´e´.

ecaa´ce´ —> ecaa´c´e´

Figure IV

There are two points b radiating from point O. The lower is likely to represent b´.

Chapter 5:

inportant difference between —> important difference between