  The ab-plane optical spectra of one underdoped and one nearly optimally doped
single crystal of HgBa_2Ca_2Cu_3O_{8+delta} were investigated in the frequency
range from 40 to 40,000 cm^-1. The frequency dependent scattering rate was
obtained by Kramers Kronig analysis of the reflectance. Both crystals have a
scattering rate gap of about 1000 cm^-1 which is much larger than the 700 cm^-1
gap seen in optical studies of several cuprates with maximum Tc around 93 K.
There appears to be a universal scaling between scattering rate gap and maximum
Tc for the cuprate superconductors.
